I've found a bug in version 9.1. I've looked through the past couple pages of this thread and didn't see it being previously reported, but I could have missed it.
When two bots are set to place an object into a currently-full storage, and you take one object out of the storage, only one succeeds in placing the object in storage, but they both move to the next task in their program, so the other bot is still holding an object and its program gets thrown off. (My two bots are at separate storage boxes, in case that's important.)