What’s the most common mistake micro-stakes cash players repeat?

They do not study all the way thru a hand. They spend hours studying preflop when the pot is the smallest then study flops when the pot is medium size and spend very little time studying what to do on turns and rivers when the pot is huge.

When you study your hands study what would I do if Villain called flop lead turn XR turn even if your hand did not go pass the turn. then estimate what cards on river would make you take what actions.

Limp
Not 3bet enough
Call 3bets too wide
Too passive post flop
Not good at evaluating relative hand strength
Tilt

...and many more, which is why the games are so soft

A lot of mistakes, but one of the most common is thinking: "Oh, I have $10, I'll turn that into $100 by playing some NL2 tournaments or even micro stakes tournaments."

Spoiler: No, you won't! Variance may help you sometimes, but it's still risky...

I'm talking about bankroll management, which can ruin even professional players if they ignore it.

Playing too many hands.
Most players don’t lose because they bluff too much.
They lose because they enter pots with weak hands, out of position, with no real plan.
At micro stakes, patience is a superpower.
You don’t need to win every pot you just need to stop donating chips in bad ones.