Ooh, this is an interesting thread. I've been doing translation for... a while, and here's some tips I usually share with new translators that cover some of the common mistakes:
For translators:
1.)
Not double-checking any number greater than 4. It is incredibly easy to write down the wrong number, and you look like a dumbass when you do it. All you have to do is accidentally press the key one over to the left of what you meant to hit, and your editor/QC will never catch it. There's nothing worse than writing down "The King's Guard has twelve ultra-mechs." and having mechs 13 and 14 show up and start trashing the heroes in the next episode.
2.)
Not reading the show's website/wiki. In 2012, every show, no matter how small, will have a website and Japanese wikipedia article. These include story summaries, character profiles, and most importantly, vocabulary lists. They're also particularly helpful for in media res first episodes, where it's expected that the viewer won't know the meaning of every word or the significance of what's going on. As the translator, you have to have make sure you have as solid an understanding as possible or you're going to make a mess of things. And if you're working off an established property, there are often ENGLISH wikis devoted to the show like
this one or
this one. I don't know how I would've made it through the third season of Shakugan no Shana without that latter.
3.)
Not double-checking when something doesn't make sense. This is very important, and unfortunately it's a skill you either have or you don't. If something seems strange, if the heroine says something totally out of character or that totally contradicts the plot up to this point, DOUBLE-CHECK IT. More often than not, you've messed something up.
For editors:
If you ever find yourself saying the words "I don't think this character would say this, so I'm going to rewrite it," then hit yourself in the face with a ball-peen hammer and save your translator the trouble. Particularly if it's in episode 3 of a show.