If there are multiple unclosed tags of the same kind where the effects accumulate (ex: small, big), Tidy and Remex behaviors are different. With Remex the effects accumulate with each unclosed tag whereas with Tidy, it seems to be capped at 1 or 2 depending on where the unclosed tags show up.
The most dramatic effect of this can be seen on this itwiki page.
So, any page with 2+ unclosed small or 2+ unclosed big tags should be flagged. There can be some false positives in terms of differences, but better safe than sorry.