Well, the things I always come across are two:
English expressions which subbers are not familiar with and therefore translate them incorrectly.
This list contains several of them.
The ones that stand out the most are: “It can’t be helped” and “to stand someone up”.
I have seen so many people translate them literally and it’s sad to see that because I can edit them when I moderate but what about the countless shows that have a moderator who doesn’t know? It will be the same wrong wording over and over again.
Also, the expression “can tell.” which -as far as I know- is supposed to mean that something is distinguishable. For example: “I can tell that he’s upset about…” or “Whether it’s this or that, no one can tell.”
I hope I did not make any mistakes of my own in this post. 
The other mistake is a grammatical one so I don’t think many people will relate to it here since this is an all-language discussion.