what doesn't go on beans'n'rice?

Hakarl, probably.

For a non-joke answer, I'd say cheese. I know cheese on rice is a thing in some places, but every time I've had it it's been very unpleasant, like the texture and flavor just clash so horribly. A jam would be weird too, and probably worse than just mixing in whatever fruit it was made from. Mashed potatoes would probably be a weird blend, and worse than mixing in chunks/sticks of fried potato.

I guess most of those are more like "if you prepare this blend of ingredients wrong it won't be good" than ingredients clashing, though, apart from the cheese one. So that's what I'm going with: cheese is not a good pairing with rice and beans.