I do it when I take a pic with my Mom's or a couple other people's phones, some people do not even care that they put their fingers on the lens, and some phones are just made in such a way that it's hard to avoid if you're not thinking about it.

Edit: I have had it happen to me a few times, I dislike calls with the phone pressed against my ear with a passion so I tend to avoid it, but I feel like that makes it more likely to smudge the lens so if you do it often.