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For example?
SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)
In Smite the goddess Hel is seen using the weapon of another god. How she gained and why she uses it is unexplained.
I can't find anything quite right, but this trope doesn't sound weapon related, it's more "Something is out of place that seems to violate Offscreen Inertia (i.e. obviously something must have happened off-screen because stuff isn't how it was the last time we checked in), but it's never commented on or explained." The closest I've seen is What Happened to the Mouse?, but that's "A plot point is dropped and never heard from again" rather than "Obviously a plot point happened but no one talks about it".
You might find something in Continuity Tropes or This Index Happened Offscreen, but I didn't seen anything. It might be worth taking to YKTTW.
SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)
Alright thanks.
What's the trope in which a character is seen using a highly unexpected weapon unrelated to them, sometimes making it even surprising for readers/players.