i had a similar experience with a sidewinder i had attached to gt175 briefly. luckily at low speed, no real damage. i took it off and replaced it for a 125 sticker.
Ouch! Coming back from France a while ago on a very old boat from Cherbourg to Portsmouth, one with pillars on the car deck. A red VFR800 with panniers and pillion came onto the deck, tried to manoeuvre past one of the pillars... Well you can guess the rest. One smashed pannier later, bike on its side, pillion on the floor and an almighty argument broke out between rider and pillion. Apparently not the first time he had none that. Didn't sound like the rest of the holiday had gone very well, either.
He was lucky that he landed in the boat and that the bike only hit the piers/ floating dock. That fibreglass yacht would have likely suffered massive damage if the bike had gone into the cockpit...
I had my pannier taken off once, when riding in a group. Followed the leader in to a parking spot and stopped waiting for pillion to get off. Another rider behind came in on a borrowed bike, forgot he had panniers on it and came too close to me and dinged my case down the lay-by, bouncy-bouncy. His stayed on. My wife on the back wasn't too happy as it came off on her recently broken leg side, but no further damage done. Cable ties and tape repair to last the rest of the tour.