That error's had me stumped for a while as well—I've never encountered it before, and haven't been successful in my attempts to replicate it either, so any info is useful in figuring it out.
In the code, the "jamp_1"-and-so-on strings are set when you click on an item from Stuff after being prompted. These strings take the name of the label you were in when you triggered the stuff prompt, and then append ".req1" (and so on). In your case (the key on the string), the jamps should have been set as "check_parlor_1.req_1", and so on.
When working correctly, the jamps are then used to route the game to the correct outcome of your Stuff selection. They're not used at all if there's no unique dialogue for the selected Stuff, and each Stuff prompt has unique dialogue for only 3 of the 6 Stuffs, which would be why How 2 Metric doesn't cause the error. If you were interacting with, for example, the mirror in the bathroom, then 1, 2, and 3 would correspond to the mask, book, and gun, and the other three Stuffs would give you the generic message.
If the jamps were set with "_return" instead of "check_parlor_1", it means that the system that registers the current label encountered a bug somewhere. Unfortunately, that's as far as I've gotten—I suspect it has something to do with internal labels that Ren'py uses, but I'm not certain.
At present, my first suggestion would be to go to the bathroom, use the gun on the mirror, and then attempt to get the key again. If the problem persists, that would mean there's something wrong with that particular Stuff prompt, which would narrow it down considerably.
I'm really sorry for the inconvenience! Please let me know if you continue encountering this error.