When I do fixup commits, I often think there will be no conflict when it gets rebased but it turns out there is. Do you not worry about mistakes entering at this (post-review) stage?
Yes. It has happened especially as the team has scaled in developer size and when we are in development modes where there's better chances for conflicts, for example iterating on core code during feature development, or when we're touching/adding to common configuration settings. But it hasn't been that often where a developer needed to fix a tricky conflict.
I encourage members on our team to rebase frequently as they're iterating prior to opening their PRs for their individual feature branches and I think this helps us avoid problems for our typical PRs.
Also as a rule, don't sign off on a PR until all conflicts are resolved, if there were any and review the final commit(s) carefully.
We also have the option because of some of the CI tools we've written to give our QA people access to specific SHAs that can be tested, so sometimes (rarely) QA sign-off is asked for as part of the PR.
