Hahaha, we saw the setlock scene where John and Sherlock are hurrying to that carriage and Sherlock calls him by his first name, right?
How weird (and awesome) would it be if this really was just by accident???
In the heat of the moment John probably wouldn’t care, but then, sitting inside the carriage, Sherlock would probably continue brabbling about the case and John sitting there “You have a girlfriend”-HLV-style doing the eye shifty thing and suddenly burst out, “… Did you just call me ‘John’?” And Sherlock would have the shock of his life, stammering, and John sitting there, bewildered but also a bit amused, avoiding eye contact, “No… it’s fine. S-Sherlock.” And Sherlock suddenly dead silent just nods and they sit there not knowing how to handle what just has happened, but also neither of them being mad about it.