Being a tattoo apprentice, and learning new things every day, you are bound to mess up every once in a while. The mistakes are what teach you what you need to know. BUT, there are some things a tattoo apprentice should NEVER do.
5. Never handle unsterilized equipment. Biohazard
4. Never get a tattoo machine, or power supply wet.
3. Don't ditto the pattern backwards.
2. Don't hit on your teacher's girl (or boy).
1. Don't make your teacher look bad in front of a client.
I have personally already made some of these mistakes, they do happen. I'm just trying to make it easier on you and your teacher by making you aware.
Thanks for reading,