I think the most common mistake of a newbie are:
1. Getting scammed
2. Using faucets
3. Not buying BTC when it's low.
4. Investing in ponzis.
You gotta understand how bitcoin works as a newbie. Don't go for the quick profits, and go to the dodgy ponzi schemes. Nor should you sacrifice your time for tiny rewards on faucets.
Yeah to be honest i also do that things when i was a newbie and now i realize that's one of my stupid decision. But now i learned the lesson and i'm now using my bitcoin in a smarter way than before.