Why am I getting 20V from a light socket after changing the bulb?
BMitch's answer is most likely correct, but there is another possibility as well, although  it's unlikely. If the breaker tripped and the line feeding the socket follows parallel to another live line, then you can get a small amount of voltage on the line. They basically act like a very badly designed transformer. Although it won't have any real power behind it. I've seen this happen a few times before and with voltages as high as 70 volts. But again it's more of an odd occurrence than anything though, but something to think about if you rule out the usual suspects.