A big "oops" probably went through the slashdot office when the hit the limit of 16,777,216 comments in their system which used an unsigned mediumint as the ID type in their mysql database.
Changing the column in question apparently took three hours easily -- no wonder on a multi-million row table.
A quote:
We shall flog ourselves accordingly -- the slashdot people themselves
Or, to say it with the words of the first commenter (after the issue was solved...):
*Clap clap clap*
Golfclap for slashdot!
(via google blogoscoped)