So trying to get a consensus of how people play their games here: What do people do when they find out they got things wrong mid-game or after the game? Like, someone should have taken damage or that chaos draw should have been -4 instead of a -1. Do people keep going, go back or redo the whole campaign again?

It's a complicated game, and some errors are inevitable. If detected errors are not in the players' favor, we just keep going and play correctly moving forward. If we've made errors in our own favor, we might assess ourselves an appropriate penalty (lose a couple clues, add some doom) if our judgment allows. But it doesn't invalidate the session, let alone the campaign. This is a co-op, and we're playing for fun.

Usually, we just bodge it if we can to be as close and fair as possible. If the seeker forgot they were down 1 point of Intellect and should have failed a test, they put a clue back on that location. If I forgot to collect my Lone Wolf money at the start of last turn, I just grab an extra resource now. If the Mystic forgot that they were meant to take horror last turn because of the backlash from Shrivelling, they take the extra horror now. If this would defeat them, they get defeated now. That might mean that they got a few extra actions they shouldn't, but it might also mean we got an encounter card we shouldn't have, so it usually balances out. It gets a bit more complicated if things would have been done differently ("well if I'd known I had an extra horror on me I would have used Fearless on that Rite of Seeking test") but we generally err on the side of not letting anyone benefit from their own mistake, especially if the error is player error rather than an error with the framework of the game (if you forgot that an enemy was engaged with you and you would have provoked AoO, it's fine to undo the action if it doesn't require much rewinding, but if it's instead a turn down the line and we've all been playing on the basis of the results of that action, you just take the damage now).

If it's a profound mistake (e.g. we messed up the setup of the scenario, or forgot that an enemy was invulnerable and killed it 5 turns ago, or whatever) we just restart the scenario.