Prompts check fandom restrictions based on the set of all TagSetAssociations, instead of just the TagSetAssociations from the tag sets that were explicitly associated with the PromptRestriction for that prompt.
The character or relationship from the unrelated TagSetAssociation won't autocomplete, but the prompt will be considered valid, and won't raise an error.
Log in as User A.
Set up a gift exchange or prompt meme.
Make sure that prompts allow participants to add at least one fandom and at least one character.
You can either use a tag set with at least one fandom, or allow all canonical tags.
Make sure that you check "Fandom only?" under the "Character Settings:" section.
Log in as User B.
Open the sign-up form for User A's challenge.
Fill in a fandom from the tag set (or a canonical fandom, if you didn’t add a tag set).
Pick a non-canonical character that’s completely unrelated to the fandom. The character in question doesn’t have to be in the tag set.
Copy-paste the character name (since it won't be in the autocomplete, of course).
Note that when you submit your sign-up, it displays an error.
Go to Browse > Tags > Tag Sets > New Tag Set.
Fill in your preferred name.
Copy the fandom name into the "Add Fandoms" field.
Copy the unrelated character name into the "Add Characters" field.
Press Submit to create the tagset.
Edit the tag set, scroll down to the "Tag Associations" section, and select your character and fandom from the tag and parent fields, respectively. Save the change by pressing "Update."
Go back to the sign-up form for User A's challenge.
Fill in the fandom and character again.
Note that you no longer get an error when you submit your sign-up. You're now permitted to sign-up with that fandom/character combo, even though the only tag set associating them is one you created yourself, and the character might not even be in the tag set.