Merges can be undone. Either you can manually remediate by `jj abandon`ing the merge commit so that it's not visible anymore, or you can restore the entire repo

Merges can be undone. Either you can manually remediate by `jj abandon`ing the merge commit so that it's not visible anymore, or you can restore the entire repo state to a previous point in time with `jj undo` or `jj op restore`, or you can do some remediation in between those two extremes. Off of the top of my head, `jj new` and occasionally `jj rebase` can create merge commits; I don't recall any others.