Right. But that can happen accidentally. But per the terms it doesn't matter whether it was an accident or not.
By way of example, let's say that hypothetically some court says that interfaces are copyrightable, like Oracle claimed for Java. All of a sudden that contribution of code implementing the Java API is infringing. Or rather, it was always infringing, but only now do you know that.
Or you're an employee of a software company. The free software code you wrote happens to be so related to your company's business interests that it falls within the scope of your employment, so the code is theirs. The FSF probably tries to screen these people in their questionnaire, but they can't always be right about it.
There are tremendous grey areas in copyright law, and the FSF clause shifts most of the repercussions of this onto you. That's its very purpose. If they didn't want to do this, they would have qualified it with a variant of something like "known or should have known". The Ubuntu language, "to the best of my knowledge", is even more pro-contributor.
Also consider that you're indemnifying for any "alleged breach." So maybe you were in the right, but the FSF chose to settle out of prudence. You're still on the hook. But now let's say you can find a way around the language, or that my interpretation is totally bogus. My interpretation isn't so bogus that you won't be forced to spend thousands of dollars on an attorney to make your case.
But now let's say that we all agree none of the FSF officers would ever willing do any of this. (Which probably goes without saying.) I don't know much about non-profit governance, but if it were a for-profit corporation the benevolence of the officers doesn't matter much. If they don't sue they could possibly be on the hook for shirking their duty to protect the corporation.
None of this is worth discussing anymore, though. Frankly, I was just surprised that these organizations would use this language, and even more surprised that Ubuntu didn't. In reality we all take much greater legal risks every day.