What’s the most expensive “small mistake” a Bitcoin beginner can make?

I’m not asking about obvious stuff like “don’t invest money you can’t afford to lose.”

I mean the smaller mistakes that look harmless at first but can become very expensive later.

Examples:

• leaving BTC on an exchange too long
• bad seed phrase storage
• buying without understanding withdrawal fees
• using the wrong wallet setup
• overcomplicating security too early
• underestimating tax / tracking issues

For those who’ve been around Bitcoin longer:
what’s the beginner mistake that looks small at first, but causes the most damage later?

Not looking for hype — just trying to learn what actually matters early.