In planning phase I think if proper care and hard work you can make a perfect Gantt chart but one thing to almost impossible to get correct in it is to cover all pitfalls. So what are the ways you can think off that can cause pitfalls and also come up with a solution to avoid it.

According to me availability of team member is one of the biggest uncertainty as project manager which can be reduced if we ask to everyone related to project to give anticipated days for which they wont be there or wont be working so we have buffer accordingly while planning.