When adding new waypoint (not stops) to a schedule for a line (of road vehicles), some vehicles end up stuck when arriving at the next stop. They show the text "Loading (100%->0%)!" I have reported something similar earlier, but I think that happened in a different situation and was fixed.
I haven't been able to figure out exactly what situation triggers it, such as whether the vehicles that get stuck have started on the leg before it got a new waypoint or whether they have passed where the new waypoint is. It might also have something to say whether there were other waypoints on the leg before a new one was added. (Waypoints were never removed.) I might be able to figure out more details for how to reproduce at some point, but I'm posting this now in case what I've written is enough for someone to figure it out.
Not sure if it's the same bug... In the attached, the bus has arrived at the stop (item 2 in the schedule), however it still thinks it's enroute to the waypoint in the tile immediately before. Hence it doesn't load/unload and remains at the stop forever (ignoring the max wait time in the schedule for the stop).
As the waypoint and the stop are only a single tile aparts, I would guess the same coordinate was added twice at the start or so, or the way search was deemed unsuccessful. So I think this is a bug.
EDIT: should be fixed in r7613