I did not know that. It reminds me of how Castlevania: Rondo of Blood has a special stage (with a graphical style very much *not* what you would expect in a Castlevania game) that you can play if you don't have the proper upgrade card inserted.

Another one: In Castlevania: Harmony of Dissonace, after unlocking Boss Rush mode, enter the Konami code on the Konami logo. If you then play boss rush mode, you will be playing as Simon Belmont (from the original Castlevania), complete with a Vampire Killer remix not otherwise heard outside the sound test.

Don't forget that Final Fantasy 4 and Chrono Trigger both have deveopler rooms in which you can actually fight some of the members in them.

They won't make that mistake again...

Is there difference between what we're posting and Easter Eggs?

I've got plenty of those to mention but hidden levels, skill sets, maps seem to go way beyond just Eggs and I don't want to sully those mentions with "Hey look, a panda!" :P

Warcraft 3, or how an easter egg can turn terribly wrong ;)

Was WC 3 the one that also had the Fight Club reference?

Hmm, I don’t know this about one…

The cow level!

Not sure if it counts, but I've spent hours - what am I saying, DAYS - clicking on the photographer's bottom, in Infogrames' "North & South".