Alex at The Daily WTF wrote a great article on this subject.
http://thedailywtf.com/Articles/Database-Changes-Done-Right....
http://thedailywtf.com/Articles/Database-Changes-Done-Right....
(Or if you're really lucky, even the current load on the database could make it fail... i.e. transaction serialization failure due to the random row order you attempted to lock)
So it should have ended with 'you're doomed'.