What's the biggest strategic mistake you see recreational players make ?

[Scorpion Girl]

I'm thinking about those habits that seem harmless or even logical to beginners but are actually slowly draining their bankroll over time. Is it things like playing too many hands preflop, chasing draws with bad pot odds,or not adjusting to different opponent types? Or maybe it's more subtle stuff like poor position awareness, calling too much instead of raising or folding or not paying attention to stack sizes? I feel like a lot of recreational players focus on learning fancy moves or reading tells,but miss fundamental mistakes that are way more costly. What's that one thing where you see someone do it and immediately know they're leaving money on the table?