Easter eggs are a common piece of software (see: [[Easter egg (media)]]). To my knowledge, MediaWiki doesn't contain many Easter eggs currently, aside from parser irregularities and crude code comments.
I suppose naming the "Bad image list" as Tim did is sort of an Easter egg (see: T16281#192741), but that's a bit too obscure for my tastes.
A neat trick or hidden feature of the software would be nice. Apparently additional languages are a no-go due to ISO rigidness (see: r55092).