Also any estimate providedwill be pushed downwards, not to mention unforeseen curveballs, so there is an incentive to estimate higher to preserve your own sanity. Some don't learn to estimate higher until they get burnt.

Another common pitfall is to estimate each task in isolation, and the people receiving the estimates considering them timelines. Effort estimates and delivery timelines are wildly different beasts and great care has to be taken to avoid miscommunications.