In general, yes.
Nobody should get defensive if you're tactful about it. An easy way to handle it is to ask them to double-check your change before you commit it back to the trunk (or whatever is relevant for your version control system). People will appreciate it if you save them a few minutes by fixing obvious errors, but they won't appreciate it if you fix something that wasn't broken and end up breaking their code. Giving them a chance to review your change tells them that you don't want to step on their toes and gives them an opportunity to object to your changes.
If it's a big change rather than just a typo, it's a good idea to give the author a heads up before you dig into trying to fix it. "Joe, I was merging my own stuff yesterday and found something that I'm not sure I understand. It looks like a bug, but I wanted to run it by you before I go messing with your code. Would you take a look with me?"
Your relationship with the author is a big factor. If you wouldn't mind the author fixing your code without telling you, and if you're pretty sure the feeling is mutual, then it might not be worth mentioning. If it's someone with more experience/seniority/status, you'll want to let them know that you're going to change their code. If it's someone with less, then consider whether it's the sort of thing they need to hear to avoid repeating the mistake or it might embarrass them needlessly.
Always remember that if you can find out who checked in the "bug", they can as easily find out who "fixed" their code. If you think they'd be upset/annoyed/embarrassed at finding out about your change after the fact, by all means tell them beforehand.
Also, fixing the bug isn't your only option. You can always just report the bug in your issue tracker. Tact is again required here -- reporting the bug makes it more visible to the entire team, but it also gives the author a chance to fix his or her own mistake. Reporting is the best option if you're not certain about the best way to fix the problem or if you just don't have time to fix it.