I have no source. Idk if it actually happens that "ordinary" people go to jail for being bad at filling out taxes. But people think it happens, people worry it happens and that's completely fair.

Isn't the difference between "fraud" and "incorrectly filling out documents" basically intent? And intent is hard to prove or disprove.

Yes, so there's no reason for people to think they will risk going to jail for incorrectly filling out their tax returns.

Yeah, you won't go to jail for making a mistake. Penalties and interest at the worst, and those (in my experience) are waived if you ask nicely.