How do you quote bug fixes on messy legacy code you haven't seen yet?

Taking on a quick bug fix for someone else's codebase always turns into a trap when you realize the underlying structure is a complete mess of outdated plugins and spaghetti code.

Quoting a flat fixed price beforehand usually means underestimating the hours, while billing hourly makes clients anxious about open-ended bills.

If you do freelance bug fixing, how do you handle estimates without either losing money or scaring off the client?