I do this for a living so I have a few words about it.

1. Obsessing over the meaning of individual words, and wrecking what the text (or dialogue) says on a discursive level. I see this all the time with Latin, but it pops up often in Japanese too - such as muppets translating "貴様" kisama as simply "you..." (literal translation) instead of something like "bastard" or "piece of shit" or whatever. Sure, "貴様" is "ackshyually" a pronoun, and then what?

2. Not paying attention to the target audience of the translation. JP→EN example again - it's fine if you keep honorific suffixes as in the original if the target audience is a bunch of weebs, we get it. But if you're subbing some anime series for a wider audience, you need to convey that info in some other way. (Don't just ditch it though, see #1.)

3. Not doing due diligence. It's 4AM, you got more work than you have time for, you need to keep pumping those translations. Poor little boy, I don't bloody care - spell-proof and grammar-proof the bloody thing dammit. "Its" for possessive, "it's" for pronoun+verb; "por que" if question, "porque" if answer; "apposto" if annexed, "a posto" if it's OK.

4. Abusing translation notes. If your "TN" has four or more lines, or the reader already expects one every single page, you're doing it wrong.