A WebmasterWorld member in the AdWords forum documented that after clicking on a link two times quickly, Google spit back a funny error message. The message read;
Give your mouse a break. It looks like you may have clicked the same link twice. Relax for a moment, give us a chance to respond to your first request, and then click your browser back button and try again. Thanks for your patience. -- a Google engineer Error 409
Now, that is funny.
Forum discussion at WebmasterWorld.