A mistake every DBA could make,having your auto-incremental primary key big enough to handle almost every thinkable 2 to the power of n, but you overlook the size of your index.
It has happened to slashdot last week:
The funniest comment is imho:
Alright, who’s the joker who posted the 16,777,216th comment?
Thanks for breaking slashdot, jerk :D