Hi Sophie things like that usually means that a trimming boundary has become malformed in some way, like too large of gaps between segments or also if it has trim curves that criss-cross back over top of themselves instead of making an orderly boundary.

Usually such things have to be "untrimmed" and retrimmed again in order to repair it, I'll take a look at your model and see if I can help you with that. I haven't taken a look at your model yet but if you also have the step _before_ you did the boolean that got the bad result if you could post that as well that might help to see what's going on.

- Michael