When reducing the durability of a tool or an item with a procedure, it doesn't break when reaching 0/below 0. It just keeps going into the negatives.
I COULD be doing something wrong however, but I don't think so. I've attached a picture of the procedure in case I AM doing it wrong. When I tried it with a tool it was set to a special tool.
Also, since a user can only open one issue per day, I would also like to say there should be a way to detect item damage values in procedures (if there is, I can't find it.. :( )