Sort of hard to pick just one but the worst is probably Misleading or incorrect translations, secondary Bad English or grammar - these two are usually tied together anyway. Sometimes I actually have to machine translate it myself to get a sentence that makes a lot more sense than the "translated" one.

The poor font choice and typesetting is one that could be easiest fixed so that's just extra pain tho. As ex-editor myself it just hurts seeing. 🤨 Better cleaning would probably require slightly more skill so I can forgive, though once it's like extreme overleveling on low quality artifacted stamped across every panel image.... hahaha. Well even being able to clean well would probably not save that.

SFX....even the readable scans from the speed release groups rarely translate them so I kind of stopped expecting it. However, what I hate the most is when they straight up skip bubble or small text.

Please never die, MangaUpdates.