Thats all too funny, since thats the general rule of thumb for software estimates (not really, but jokingly it is).

To go a little off topic, this rule is carried forward when the estimate goes "up the chain of command" as well.

My manager asks how long something will take, and I think, "I can do this in a day", so applying the rule, i tell him 2 weeks.

He has to report this to his manager, our director, and following the rule, he tells him, 4 months.

Now of course, this needs to go up to our divisions VP, and again following the rule, he tells him 8 years.

Of course the VP is furious, and what would have been a simple change, gets canceled.