It could mess them up, because in theory a single wrong answer with 100% confidence renders the entire series infinitely poorly calibrated. The survey says that this won't be done, that 100% will be treated as something slightly less than that. But how much less could depend on assumptions that the survey-makers made about how often people would answer this way, and maybe I did it too much.

I doubt it, since I'm pretty sure that they know enough about these pitfalls to avoid them. But I felt that I answered 0 and 100 quite a lot, so I thought that some warning was in order.