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.
1. I do not get scammed anything, because I do not make investments
2. use the faucet for newbie I thought its not wrong and still worth
3. I do not buy BTC when low, because I did not have the money to buy
4. I have read that site ponzi investment is very dangerous, so I stay away from it