As I explored Thorne Manor again, enjoying how well the random sound generation actually worked out, I couldn't remember what clever refusal message I had implemented if someone tries to pick up the bathtub:
With all the poorly-conceived inventory limits I had initially put in that game, my photographer was now walking around a haunted house carrying an entire claw-foot bathtub with shower and curtain. Luckily I had restricted objects that could be put in the PC's coat pocket with an adjective, so I was spared that ridiculousness, and I kept it in inventory since I didn't want to see if the butler would dutifully tidy it up and shut it in the hall closet. I solicited a lot of feedback post-comp from experienced beta testers, and nobody (including me) had ever caught this very easily fixed bug.
So what's the longest a bug has gone undocumented in a game? Comment below and tell me about the glitch you found years later in an old Comp entry or that one bit of weirdness that the Infocom team totally missed. Or anything obscure and hilarious that crops up when an IF is abused beyond what the author considered.