diff --git a/lib/puppet/util/pidlock.rb b/lib/puppet/util/pidlock.rb
index fcf0cf296..a0fe99e89 100644
--- a/lib/puppet/util/pidlock.rb
+++ b/lib/puppet/util/pidlock.rb
@@ -1,72 +1,117 @@
 require 'fileutils'
 
 class Puppet::Util::Pidlock
   attr_reader :lockfile
 
   def initialize(lockfile)
     @lockfile = lockfile
   end
 
   def locked?
     clear_if_stale
+    return true if File.exists? @lockfile
+
+    # HACK!  There was a temporary change to the lockfile behavior introduced in 2.7.10 and 2.7.11, and reverted
+    # in 2.7.12.  We need to pull some chicanery to be backwards-compatible with those versions.  For more info,
+    # see the comments on the method... and this hack should be removed for the 3.x series.
+    handle_2_7_10_disabled_lockfile
     File.exists? @lockfile
   end
 
   def mine?
     Process.pid == lock_pid
   end
 
   def anonymous?
     return false unless File.exists?(@lockfile)
     File.read(@lockfile) == ""
   end
 
   def lock(opts = {})
     opts = {:anonymous => false}.merge(opts)
 
     if locked?
       mine?
     else
       if opts[:anonymous]
         File.open(@lockfile, 'w') { |fd| true }
       else
         File.open(@lockfile, "w") { |fd| fd.write(Process.pid) }
       end
       true
     end
   end
 
   def unlock(opts = {})
+    return false unless locked?
+
     opts = {:anonymous => false}.merge(opts)
 
     if mine? or (opts[:anonymous] and anonymous?)
       File.unlink(@lockfile)
       true
     else
       false
     end
   end
 
   private
   def lock_pid
     if File.exists? @lockfile
       File.read(@lockfile).to_i
     else
       nil
     end
   end
 
   def clear_if_stale
     return if lock_pid.nil?
 
     errors = [Errno::ESRCH]
     # Process::Error can only happen, and is only defined, on Windows
     errors << Process::Error if defined? Process::Error
 
     begin
       Process.kill(0, lock_pid)
     rescue *errors
       File.unlink(@lockfile)
     end
   end
+
+
+  ######################################################################################
+  # Backwards compatibility hack
+  ######################################################################################
+  # A change to lockfile behavior was introduced in 2.7.10 and 2.7.11; basically,
+  # instead of using a single lockfile to indicate both administrative disabling of
+  # the agent *and* the case where an agent run is already in progress, we started using
+  # two separate lockfiles: the 'normal' one for the "run in progress" case, and a
+  # separate one with a ".disabled" extension to indicate administrative disabling.
+  #
+  # This was determined to cause incompatibilities with mcollective, so the behavior
+  # was reverted for 2.7.12.  Unfortunately this leaves the possibility that someone
+  # may have run "agent --disable" to administratively disable a 2.7.10 or 2.7.11
+  # agent, and then upgraded to a newer version.  This method exists only to
+  # provide backwards compatibility.  Basically, it just recognizes the 2.7.10/2.7.11
+  # ".disabled" lock file, warns, and cleans it up.
+  #
+  # This should be removed for the 3.x series.
+  #
+  # For more information, please see tickets #12844, #3757, #4836, and #11057
+  #
+  # -- cprice 2012-03-01
+  #
+  def handle_2_7_10_disabled_lockfile
+    disabled_lockfile_path = @lockfile + ".disabled"
+    if (File.exists?(disabled_lockfile_path))
+      Puppet.warning("Found special lockfile '#{disabled_lockfile_path}'; this file was " +
+          "generated by a call to 'puppet agent --disable' in puppet 2.7.10 or 2.7.11. " +
+          "The expected lockfile path is '#{@lockfile}'; renaming the lock file.")
+      File.rename(disabled_lockfile_path, @lockfile)
+    end
+  end
+  private :handle_2_7_10_disabled_lockfile
+  ######################################################################################
+  # End backwards compatibility hack
+  ######################################################################################
 end
diff --git a/spec/unit/agent_backward_compatibility_spec.rb b/spec/unit/agent_backward_compatibility_spec.rb
new file mode 100644
index 000000000..1cd587fa1
--- /dev/null
+++ b/spec/unit/agent_backward_compatibility_spec.rb
@@ -0,0 +1,154 @@
+#!/usr/bin/env rspec
+require 'spec_helper'
+require 'puppet/agent'
+
+
+############################################################################
+#                                  NOTE                                    #
+############################################################################
+#                                                                          #
+# This entire spec is only here for backwards compatibility from 2.7.12+   #
+# with 2.7.10 and 2.7.11. The entire file should be able to be removed     #
+# for the 3.x series.                                                      #
+#                                                                          #
+# For more info, see the comments on the #handle_2_7_10_disabled_lockfile  #
+# method in pidlock.rb                                                     #
+#                                                                          #
+# --cprice 2012-03-01                                                      #
+############################################################################
+
+class AgentTestClient
+  def run
+    # no-op
+  end
+  def stop
+    # no-op
+  end
+end
+
+describe Puppet::Agent do
+  include PuppetSpec::Files
+
+  let(:agent) { Puppet::Agent.new(AgentTestClient) }
+
+  describe "in order to be backwards-compatibility with versions 2.7.10 and 2.7.11" do
+
+    describe "when the 2.7.10/2.7.11 'disabled' lockfile exists" do
+
+      # the "normal" lockfile
+      let(:lockfile_path) { tmpfile("agent_spec_lockfile") }
+
+      # the 2.7.10/2.7.11 "disabled" lockfile
+      # (can't use PuppetSpec::Files.tmpfile here because we need the ".disabled" file to have *exactly* the same
+      #   path/name as the original file, plus the ".disabled" suffix.)
+      let(:disabled_lockfile_path) { lockfile_path + ".disabled" }
+
+      # some regexes to match log messages
+      let(:warning_regex) { /^Found special lockfile '#{disabled_lockfile_path}'.*renaming/ }
+      let(:run_in_progress_regex) { /^Run of .* already in progress; skipping/ }
+
+      before(:each) do
+        # create the 2.7.10 "disable" lockfile.
+        FileUtils.touch(disabled_lockfile_path)
+
+        # stub in our temp lockfile path.
+        AgentTestClient.expects(:lockfile_path).returns lockfile_path
+      end
+
+      after(:each) do
+        # manually clean up the files that we didn't create via PuppetSpec::Files.tmpfile
+        begin
+          File.unlink(disabled_lockfile_path)
+        rescue Errno::ENOENT
+          # some of the tests expect for the agent code to take care of deleting this file,
+          # so it may (validly) not exist.
+        end
+      end
+
+      describe "when the 'regular' lockfile also exists" do
+        # the logic here is that if a 'regular' lockfile already exists, then there is some state that the
+        # current version of puppet is responsible for dealing with.  All of the tests in this block are
+        # simply here to make sure that our backwards-compatibility hack does *not* interfere with this.
+        #
+        # Even if the ".disabled" lockfile exists--it can be dealt with at another time, when puppet is
+        # in *exactly* the state that we want it to be in (mostly meaning that the 'regular' lockfile
+        # does not exist.)
+
+        before(:each) do
+          # create the "regular" lockfile
+          FileUtils.touch(lockfile_path)
+        end
+
+        it "should be recognized as 'running'" do
+          agent.should be_running
+        end
+
+        it "should not try to start a new agent run" do
+          AgentTestClient.expects(:new).never
+          Puppet.expects(:notice).with { |msg| msg =~ run_in_progress_regex  }
+
+          agent.run
+        end
+
+        it "should not delete the 2.7.10/2.7.11 lockfile" do
+          agent.run
+
+          File.exists?(disabled_lockfile_path).should == true
+        end
+
+        it "should not print the warning message" do
+          Puppet.expects(:warning).with {|msg| msg =~ warning_regex } .never
+
+          agent.run
+        end
+      end
+
+      describe "when the 'regular' lockfile does not exist" do
+        # this block of tests is for actually testing the backwards compatibility hack.  This
+        # is where we're in a clean state and we know it's safe(r) to muck with the lockfile
+        # situation.
+
+        it "should recognize that the agent is disabled" do
+          # TODO change this to use "be_disabled" when that is re-introduced.
+          agent.should be_running
+        end
+
+        describe "when an agent run is requested" do
+          it "should not try to start a new agent run" do
+            AgentTestClient.expects(:new).never
+            Puppet.expects(:notice).with { |msg| msg =~ run_in_progress_regex  }
+
+            agent.run
+          end
+
+          it "should warn, remove the 2.7.10/2.7.11 lockfile, and create the 'normal' lockfile" do
+            Puppet.expects(:warning).with { |msg| msg =~ warning_regex }
+
+            agent.run
+
+            File.exists?(disabled_lockfile_path).should == false
+            File.exists?(lockfile_path).should == true
+          end
+        end
+
+        describe "when running --enable" do
+          it "should recognize that the agent is disabled" do
+            # TODO change this to use "be_disabled" when that is re-introduced.
+            agent.should be_running
+          end
+
+          it "should warn and clean up the 2.7.10/2.7.11 lockfile" do
+            Puppet.expects(:warning).with { |msg| msg =~ warning_regex }
+
+            agent.enable
+
+            File.exists?(disabled_lockfile_path).should == false
+            File.exists?(lockfile_path).should == false
+          end
+        end
+      end
+    end
+  end
+
+
+end