Newbies mistake which I have seen are common in this forum are :

1. Newbies asking for loans of big amounts without offering proper collateral (This gives them an immediate Red Trust)
2. Newbies think of using faucetbox and clicking faucets will make them some BTC (Wrong, as they will lose a lot of time for just a few pennies)
3. Newbies being lured from some scammers to join under them in ponzi cloud mining (Wrong as newbies will never get their investment back, while they will give a power boost to the scammer to make ROI)

Pretty much these are the most basic mistakes I have seen here, but this list is far from complete.