Board Thread:Off-Topic Discussion/@comment-14302873-20130803025840/@comment-3076833-20131002224342

"Crack pairings" are pairings that are very unlikely to work under normal circumstances due to conflicts in 2 characters personalities or circumstances.

Right enough there's no canon pairings in Vocaloid but they wouldn't be defined as crack either as it's possible for any of them to be paired up (because of songs and the like).