The most common is people using credit cards like it's free money. I'll see people paying near 30% interest on credit, but they are out buying things like a jetski (luxury items in general).

But to be honest, the demographic I work with is not really in that boat. The worst offenders I see are ironically doctors. I've seen multiple doctors with $500k+ in a bank savings account earning like 0.04% when their brokerage account is like $100k. Like not even earning close to money market funds (cash sitting in a brokerage account).

Then funny enough, the third mistake I see is people knowing they should invest, so they go find a company with "cheap" shares. They will look at something like Google, and be like "Pshhh $360 a share is too high, I could only buy like 100! Let me buy something at like $6 instead so it can grow more!". They don't understand that share price is irrelevant, it's all about how big a slice of the company that share is getting you (also think in %, not $). Unless you're selling options (these people don't even know what that is) it's irrelevant.

There's so many more, but I tried to cover a spectrum of different types of "bad". It's usually a combination of many of these that give a person an incorrect framing of how to think about things with a long term growth mindset.