It depends a bit I guess. I put once. If it was early on, I could let it slide without much issue. If it's my T that I've been seeing for 5 years and had always the same session time and day, I would at the very least freak out a lot, and depending on the explanation I couldn't even forgive once.
I'm generally very well organized and know my schedule. I can understand an honest mistake one time, but if it's more than once I'd just get stressed out way too much.