Well, there's one unwritten rule that hasn't been mentioned. If you arrive late, at least recognize the inconvenience and make your apologies - sincerely! Then, you will still be required, deservedly, to accept a certain amount of ribbing, even if being late was due to circumstances outside of your control.
Last spring, a group of us were late a good 4 weeks in a row, some for real unforeseen reasons outside of our control, some for admittedly poor planning. For example, someone having to stop for breakfast and gas, without having factored that into our time or not having included a Late Ray factor. Anyhow, we were at least appreciative that they waited and we finally learnt to tack on an extra 20 minutes for the unforeseen, though I'm sure that Bill and Larry might put the emphasis on the word finally! So, I know what I'm talking about..
CdB is pretty notorious for the phaffing about, though, and when calculating a return time, I have a CdB factor that I tack on for CdB trips - I guess I should call it the phaffing factor now. I think that part of paddling is realizing that time back into town will be unpredictable and planning around that. I would imagine that the unwritten rule there, is that if you have to be back in town for a certain hour, state it at the start of the trip to make sure that arrangements are made to make it happen without inconveniencing others.
When that time comes that Bill and Larry are late, I will certainly make fun of them!! I've been waiting for years now, but it will come....One of these days, it will come...