Normally an error given when indicating that the client was able to communicate with the server, but the server either could not find the file that was requested, or was unwilling to fulfil the request for it and did not wish to reveal the reason why.
More recently used to describe a user or individual that, at any given moment will lock up and seem to have no intelligence what-so-ever and will not co-operate to solve an issue. (ie No Intelligence Found)

Adjective for something that definitely wasn't lame, but not quite "hot". In other words, slightly below being hot, therefore warm, like a nice pair of mittens.

The screen of death

404 Not Found is the completely fucking retarded screen that comes up whenever you're looking for a good porn trailer.