Everyone has their own story about objects in contemporary art museums being mistaken for art like this fire extinguisher (and even then you have to watch out for tricksters like Glen Hayward who create art objects that look like objects that might be art, like a carved fire extinguisher, albeit with reversed writing on the label).
But the other side of the is-it-isn’t-it dilemma is mistaking art for something else in an art museum. We had this happen to us, well rather to our son who was about four at the time. It happen in the Whitney Museum where seeing a large brass couch-like structure he clambered up and sat down. Ok, the cushion seemed to be missing, but hey, it was an art gallery. Five seconds later he was subjected to a right bollocking as one of the guards told him to get his sorry butt up off of the Donald Judd sculpture.
From that day on he had a deep-seated suspicion of anything that looked like it might have any sort of practical function in an art museum, like fire extinguishers, for instance.