You might not want to squash everything into one commit before merging, you can still have multiple commits in one (fast-forward) merge, as long as each of them is in a good state. This is made relatively easy by using the `--autosquash` feature of `git rebase`.

One issue I've seen a few times is that some commit in the middle of a topic branch is the problem, but if you didn't rebase it then that commit itself would look fine on top of the topic branch's parent. However, after merging it's now also on top of other commits, and the interaction with those was the problem. That makes it very hard to find such a problem. Rebasing the history of the topic branch before merging will make finding it much easier.