Is there any proof that leaving a toaster plugged in is a fire hazard?

Yes, leaving a toaster plugged in can be a fire hazard. If the toaster has a malfunction or a fault in the electrical system, it can overheat and potentially ignite surrounding materials. Additionally, if crumbs or debris accumulate inside the toaster, they can catch fire if the appliance is accidentally activated. Therefore, it's generally recommended to unplug appliances like toasters when not in use to minimize these risks.