Some of these may have been said before, but...
Mametchi/Mimitchi and Tarakotchi (and that's what happened with my Tama...at least the kid's not mutated. I think.)
Memetchi and anyone (the eyes...THE EYES!)
Maskutchi and anyone (imagine a mask on any of the other tamagotchis)
Ichigotchi and Mametchi/Mimitchi (I know this isn't possible, but think about it: A strawberry with rabbit ears... *whimper*)
Tarakotchi and Hanatchi (big lips, big nose...Gah.)