The actual duration (costs) of an activity will be about pi (3.1415...) bigger than the original estimate, even if the estimator was aware of this rule.

~Lev Virine

I like this one because... Hmmmm well, it's often true !

This rule highlights one of the numerous cognitive and motivational biases which inherently pertain to estimating and decision processes within projects lives.

If you want to know more, take some time to have a look to Lev Virine's book "Project Decisions, the art and science".