I'm not a programmer, but I wonder if it has anything to do with other systems that the inventory currently interacts with, rather than the inventory itself. That's the only thing I can think of that changed since you first made the inventory system. For example the house decorator wasn't always around, and (I assume) it modifies the inventory everytime I save changes in it. Of course this issue didn't happen to any patrons, so it's just an example. Edit: The common thread seems to be the capsule opener, which also also wasn't around when the inventory was made.

Also I can't help but focus on the fact that this happened to multiple people around the same time. Did you happen to make any recent changes on the site? Or maybe it was the result of a server hiccup?