Was on WvW today running with a havoc group. We took Durious Gulch by setting up catapults near the wall facing Umberglade camp. After taking the walls, a blackgate zerg came, we fought and then most of them left. A few had stayed and one in particular had begun to catapult our wall, using our cata. While he was on it, we could still hit him and damage the catapult but the moment he got off we could not damage it and it was ours again.
Could be hacking or a one time bug/exploit but figured it would be safe to report it anyways.
Unfortunately I have no definitive proof such as screenshots or video but I was not the only one with my guild who witnessed this.