We try to. At our company we're developing about 15 to 20 games at the same time. Some games will go well. Some games will have some challenges, as in any creative process. Sometimes you plan that one game will ship, let's say, in Q4 of this year, but there have been some changes in the business, for example, a new game has come out which changes the scope a bit, so you need to change the scope of your game.
We try to plan as much as possible but sometimes we cannot. This is a competitive industry. When your lead programmer leaves, you need a guy to replace him right away. If you don't have one, you need to find one, or you take someone from another project, but you still have an empty seat in the office that needs to be filled.