What were you doing when you got those crashes?
I got similar crashes whenever I tried to use the candle, pistol, or meterstick on the key dangling from the ceiling. The number after jamp_ and _return.req was different for different items, though.
Viewing post in Jam and the Mystery of the Mysteriously Spooky Mansion comments
Upon further investigation, the gun is 1, the meterstick is 2, and the candle is 3.
The metric book just gets a generic "I don't think this is gonna work" message, which means it must be Stuff #6! After using my Jam-like deductive reasoning, I realized that the key would be Stuff #4 and the thing inside the locked box Stuff #5, solution to the hidden puzzle wherein you find half a harpoon, which five of the endings are locked behind!
Jokes aside, a third of the game's endings are locked behind that key, and I want to enjoy them all. So I'll be tinkering with the code to find a fix. (Assuming that Ren'Py games ship with the source code, I mean.) I'll let you know if I find a fix.
EDIT: Wait, I replied that to myself. Okay, I'll let <i>the guy above you</i> know if I find a fix.
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.
Alright then. GreatWyrmGold, the world's greatest not-a-kid detective between the ages of 14 and 27 in the western half of my county, is on the case! Besides, I like having an excuse to get back into this game with its delightful sense of humor.
Test #1: Go straight from the introduction to the bedroom to get the gun, then pose with the gun in the bathroom, then...shoot the key down or something?
Result: Same crash.
Test #2: Reload save, grab the candle, pose with the gun, burn the string.
Result: "The air around this candle is hot, and according to physics, hot air rises..." No key, but I did see Jam consider the potential arson her plan could cause, which is nice. Especially since I've seen ending 16.
Test #3: Save, then try the gun just in case getting it to work makes them all work.
Result: It did! Cool.
Test #4: Save, reload other save to get meterstick.
Result: Didn't even have to re-pose before it worked. Weird, but probably useful. And definitely convenient.
Thank you for the aid in bug-testing! Your sleuthing is extremely helpful, and definitely indicates that the issue is in the label-saving code, rather than that specific interaction. Once I work out what exactly is causing the error, I'll be sure to issue a patch. Until then, please let me know if you run into any other issues, and I'll do my best to resolve them!