At the end of a short run I saved the activity, at which point the screen went blank and the watch vibrated for about 10s. It then rebooted and, on connection with my phone, uploaded the activity. First time I've ever had an unexpected reboot.
No harm done this time, but I've got a big race coming up and I would be a bit annoyed if I lost the data.
Note: Only HRM-Tri was connected