I had t2 UEF acu building T2 PDs and 1 engineer assisting acu. I decided to stop that building and reclaim that PD. ACU reclaimed PD until it got to 0 hp and then process got stuck, both ACU and engineer were trying to do something but nothing happened for that PD. It can be repeated, just set engineer to assist acu, build something and reclaim it before its finished.