I think the most common mistakes made by newbies is they easily get attracted towards gambling to multiply their bitcoins in quick time but end up loosing everything and apart from that they also invest in online sites without doing any research.
Sadly, even those who have been into bitcoin for a while are still lured into such scams from time to time. The promise of high yield in such a short period of time just seems to be good an opportunity to pass up, although they always regret investing in the end. Some of these investments are legit in the beginning but turn out to be scams because the high rate of return on investment is definitely unsustainable and can't continue until new members with incoming funds are signed up.
Yeah, plus there's no point getting into those when Bitcoin itself is the primary investment to watch anyways.