I have also seen instances where nearly an entire field is making some elementary error, which people outside that field can see more clearly, but which they can't communicate to people in that field because they would have to spend years learning enough about the field to write a paper, probably with half a year's worth of experimental work, and not get rejected, even if their insight is something that could be communicated in a single sentence.

I think that you're saying that the outsiders can't be published without learning the jargon and doing experiments. But publication is not the only avenue. If it really only takes a single sentence, the outsider should be able to find an insider who will look past jargon and data and listen to the sentence. Then the insider can tell other insiders, or tack it onto a publication, or do the new experiments.

If jargon is not just a barrier to publication, but also to communication it's a lot harder to find a sympathetic insider, but it hardly seems impossible. Also, in that situation, how can outsiders be sure they understand?

These situations sound like there is a much bigger problem than the elementary error, perhaps that the people involved just don't care about seeking truth, only about having a routine.