For me, the main mistake was buying some shitcoins hoping that they would outperform bitcoin. And to be honest some of them actually did, but it's always difficult to decide when to sell, and that also means that you need to stay behind several coins/tokens while for example if you just focus on bitcoin and maybe 3-4 altcoins maximum is way easier to keep track of everything.
Over-diversification can always be a problem. Some people would have about two dozen different cryptocurrencies in their portfolio having invested small amounts in each one of them which is completely unnecessary because you won't get a lot of profit because if some of them manage to gain good value, others might not perform so well and some might even make you lose value. So it's always better to just have a few extra cryptocurrencies apart from Bitcoin.
Newbies often make this mistake, they would invest $10 or so in a bunch of altcoins that aren't even that good only because they have seen them gaining some value when they were watching the market, they don't know their past or what project they are associated with but just made dust investments in all of them.