I’ve seen the acceleration bug (where a rider starts doing speeds up to 100km/h with normal power numbers) a couple of times in big group events but I’ve never experienced it myself.
I decided to do a BRT crit race at Richmond this morning and joined 5 minutes before the start. I selected the C cat but Zwift put me in the D cat, oh well, I’ll just take it easy then I thought. The race starts so I accelerate up to speed and then settle into a nice comfy 180w. Then I notice I’m in front of everyone else and no-one is passing me. I check my speed and I’m doing 60km/h with only 150 watts and the speed keeps going up.
I briefly consider seeing how fast I can go but figure it would be better to try fix the issue so I stop pedaling hoping that a quick stop and go would be the fix but when I start pedaling again the bug is still there. I quit the event and do a free ride in Watopia (I am on iOS which allows you to select a new ride without restarting the app) and the bug is still there. It’s only when I completely close the app and restart it that the bug goes away.
I believe this is a known issue with Zwift and I’ve sent them a support ticket about it already. Just thought I’d post this in case you ever find yourself flying along at 60+ km/h on the flat.