Ususally we have a caching backpack with spare logbooks, if we find a cache where the logbook's full.
But I have already logged two nanos myself withouth signing the logbook, because we simply didn't have the possibility to put a new logbook into those nanos (now we also carry nano logbooks around).
We are from Austria and found those caches in Budapest (Hungary), so we couldn't come back to log it.
What makes me angry ist, when you are printing out a cache and you see that there are so many logs for the owner to maintain it, because the logbook is full and the owner doesn't react. Ok, maybe he/she doesn't have time at the moment, but if I see so many "logbook full" logs, then I take a spare logbook with me. Just to be sure. If I see that at least 20 cachers have reported a missing or full logbook, then I think it is the duty of the next person who finds the cache to replace or add the logbook.