Personally, I've seen it, even when the person (especially when the person is a programmer) thinks s/he's pessimistic in estimating time, estimates are always missed. A recent example I've had was a task that the programmer promised he was going to finish it first in 2 days, I told him to be overly pessimistic, he said 2 weeks. The task was scheduled for 4. Now, after 5 weeks, the programmer says he's 2 days away from finishing.
I did publish an article about overconfidence as a strong bias in estimation (here's the link: forecasting and estimating biases in projects ).
Personally, I've seen it, even when the person (especially when the person is a programmer) thinks s/he's pessimistic in estimating time, estimates are always missed. A recent example I've had was a task that the programmer promised he was going to finish it first in 2 days, I told him to be overly pessimistic, he said 2 weeks. The task was scheduled for 4. Now, after 5 weeks, the programmer says he's 2 days away from finishing.