When whatever service I'm watching a dub on doesn't force subtitles when it should. Subtitles are still supposed to show up when there's on-screen text to translate, but sometimes they didn't do it properly and you have to keep skipping back and turning subtitles on temporarily just to get the text translation and then turn them off again. Or even worse, the service doesn't let you use subtitles at all while watching the dub and there just is no translation at all.

(Also, along these lines, it would be nice if it was standard to have both regular anime subtitles and "dubtitles" so I could watch a dub even if I'm in an environment where there's a lot of noise in the background.)