Here's one I've been noticing lately. Hopefully I can explain it well.
Bob smirked at her. "I can't help it if I have weird cravings! I'm pregnant!"
Bonnie pouted impressively at his mockery. "Fine, I'll go out and look for the ice cream, but I can't promise anything."
Bob grabbed his coat and headed for the door. "Don't come back without it!"
The blue dialogue (first and third lines) has to be Bonnie, because Bob can't be pregnant (okay, non-sci-fi, conventional story
), while the red dialogue (second line) must therefore be Bob. But from the way the narration is positioned in the same paragraph as the dialogue, you might at first think it was the other way around. Bob smirks and then complains about his pregnancy cravings? Bonnie feels mocked by this and pouts, but agrees to go get ice cream for him? It gets confusing very quickly.
With dialogue like this, I can figure out from context who's supposed to be speaking, but I've been reading lots of conversations where it wasn't that obvious, and I find it really annoying. Has anyone else seen this? Mostly I've noticed it in some fanfics lately (which are not always the best examples of formatting, granted), but it's been multiple authors, so I was afraid it was becoming a Thing...