I just found one of these last night when I was going through my wheat pennies. I thought the same thing, but then when I did some checking I saw that the mint mark wasn't upside down, it's just how the serif appears. Here is where I checked: http://www.lincolncentresource.com/San_Fransisco_Mintmark_Styles.html
Thank you, pennsteve. That is a much more helpful response than the others. I had also looked at that site, but couldn't see how a straight serif mint mark got so rounded on the bottom, but remained relatively angular at the top.
I've seen coins get worn in strange ways. How does it happen? I don't know. Maybe back when there was nothing to do except read books and gamble by tossing coins against the wall, someone used this as their lucky coin tossing penny and had a method of always holding it by the bottom with the top of the head facing the wall. That would wear down the bottom part of the mint mark if that's the part of the coin their finger always touched. Kidding of course.