Everyone makes mistakes, but as an RPer, you must know that your mistakes may be more noticeable than most. Certain other RPers may look up to you. If you play a male cannon, you may have fangirls who are watching.
Still, you can't upset them! You have to show them what you're made of! Show them your Super Saiyan power!
Of course, when you DO show your stuff- especially when you "speak"- be careful of what you say. I'm not just saying to be careful not to slip out of character in front of other people (without proper parentheses or something). I'm saying that if you stay in character for TOO long, you may say certain things that the character would say....but you take it a little too far because you've really gotten into it.
For example, if you are playing a mean character and you lash out at someone because your character bids you to...and then you realize later that it was 120% way too harsh...that's bad.
I'm not saying you can't do it, or even that it's wrong to do as an RPer. It's perfectly fine, especially if that's what keeps you in your role.
The problem is, sometimes people don't know how to snap themselves out of that state of mind once they do something extreme as their character. This is why often people say that acting is dangerous.
Please be careful.
For someone who RP's- or acts in a play, or in a show, etc etc- it is EASY to adopt that character as part of your personality. This is why I suggest playing a character that you share some personality traits with. If this doesn't help you, play a character completely OPPOSITE of you. That way, you know who is who. It's still YOU. The biggest part of being an RPer is knowing who YOU are. Remember that you may PLAY a certain character, but YOU ARE YOU.
Hope this helped <3