Relativity doesn't seem to fit at all in any reasonable way into quantum mechanics, and yet relativity is quite useful and accurate for problems at the atomic level and above.

<nitpick> General relativity doesn't seem to fit at all in any reasonable way into quantum mechanics, and it is way overkill for problems at the atomic level (and for a sizeable fraction of problems at the planetary level, too), where special relativity (which does fit with quantum mechanics, save for a few theoretical loose ends that few non-mathematicians have reasons to care about) suffices.</nitpick>