For five days now, four coders (including me) on two teams have been pouring over this terribly complicated problem. The actual size of the code involved is tiny (about 40 lines, most of which are comments), but the problem space is huge, and we have to make sure it handles every single possible case correctly.
On Friday, two of the other coders checked-in code that they concluded would work. I've just found one case on which that attempt completely breaks. I feel *amazing* right now.