So say you have a column in a database and you need it to have a timestamp in it. You think "I will use the timestamp datatype, because that is what it is for". You go ahead and do so, on your MySQL 5 installation, and it works great. The timestamp is formatted as yyyy-mm-dd hh:mm:ss. So far, so good, right?
Then you port your code and your database structure to a different server. The other server uses < MySQL 4.1. MySQL 4.1 and under format timestamps thusly: YYYYMMDDHHMMSS, which is... less useful? Man! This had me scratching my head for at least ten minutes.
Huh. Moral of the story? I now use datetime instead.