When creating a new activity and specifying a reminder date, sometimes the date setting window (img 3) doesn't open, but the slider moves across. If you attempt to save at that point you get an error message (img 1). You then have to re-click the reminder slider (img 2) to get into the date settings.

It would be a lot easier if the slider simply set the reminder date/time to the same as the activity automatically, with a hyperlink ('set') next to it to open the date settings window if you need to specify a different time.