Steve McConnell, Classic Mistakes [in software development] enumerated:
#13: Wishful thinking. I am amazed at how many problems in software development boil down to wishful thinking. How many times have you heard statements like these:
"None of the team members really believed that they could complete the project according to the schedule they were given, but they thought that maybe if everyone worked hard, and nothing went wrong, and they got a few lucky breaks, they just might be able to pull it off."
"Our team hasn't done very much work to coordinate the interfaces among the different parts of the product, but we've all been in good communication about other things, and the interfaces are relatively simple, so it'll probably take only a day or two to shake out the bugs."
Hearing things like that is very frightening.