2016 F6B: On three or four occasions in the last two years I have shifted in to first gear and then rolled slowly over a speed bump (or in the last case onto my trailer) and the bike came out of gear (not under load). I've been riding about 50 years and have owned over 40 different bikes and have never experienced this before. So, I'm thinking it likely not operator error; although anything is possible. Has anyone else experienced similar?