Not communicating with other developers on the team Not commenting code, especially business logic or view layout related Crossing MVC/other architectural boundaries where they shouldn't be Committing without running code and without testing it Coupling screens together in navigation Failing to get clarification when it's needed Not writing tests, especially for business-critical code

The more you know what you're doing, the better the estimate should be. Also padding time helps especially if you're venturing into a new domain.

Pros: Fix mistakes early (two sets of eyes) Ability to immediately bounce ideas back and forth Get to know your coworkers better Cons: Need to switch who is coding and who is observing often enough to prevent a lack of focus Personality / ego clashes can make pair programming not work for all programmers Some projects have too broad of a focus to make pair programming save time and money