The other day I put a comment in the log book but forgot to put in my name. Then there's the time I lost my pen (amongst a bunch of rocks!), the times I *forgot* my pen, and a bunch of times that the log was just too soggy to sign. There can be all kinds of reasons why someone didn't sign the log (in my case it often involves being bird-brained).
I do usually make a comment about not signing the actual log when I log it online, though.