The mistake that is often made by newbies is that they cannot maintain a private key and consider it trivial to store in a centralized wallet. Even though centralized wallets are very vulnerable to hacks.
Yes, mistake like that often happen to newbies because in their experience they also don't exist so they think the private key is not important to them, even though in general the private key is a very important code to be stored and should not be arbitrarily inputted into sites that are not important not trusted.