Physical collisions are impossible
Reminds me of an automated chemistry analysis system with a robotic archiving freezer. Every shelf spot had a corresponding sample rack with a barcode and, upon scanning the barcode, the robot arm would pull it into the -80°C freezer for later retrieval (either by rack or sample number). The low temperatures did what low temperatures do to plastics and, over time, the racks became brittle and cracked. Replacement racks were sent out and everything was gravy.
Unfortunately, one evening, a green technician couldn't remember how to use the system to retrieve the racks assigned to his team. Instead, he discovered the old racks in the storeroom and used one of these. The robot arm tried to file it and ended up wedging the old rack into the back of the new one because there was no code to check if the shelf had previously been filled or a sensor in each spot to detect the same. The loss of two racks of samples would have been annoying; instead, the robot arm managed to domino the majority of the samples off their shelves into the bottom of the freezer, smashing up the tubes and destroying a lot of work.
Fingers were pointed at everyone, by everyone. Eventually, the technician got saddled with the blame because the freezer manufacturer supplied a note with the new racks, warning that duplicate numbers should not be used in the system. All the remaining old racks were disposed of and, I believe, the software was updated (not that anyone dared to test it out).
Biting the hand that feeds IT
