Many such cases:

"It's generally accepted that defects that are found during site testing arise in four primary areas. In ascending order of impact to the schedule, they are: 

1. Areas where you thought you might find a problem, if any problems existed
2. Areas where you didn't think you'd have any problems.
3. Areas where you knew there weren't any problems. 
4. Areas where the idea of a problem was so remote that you never gave it any thought at all. " 

h/t Philip Metzger in his "Managing A Programming Project" book.