Try "Yellow Diamond's Favorite Citrine" on Fanfiction.net, its a Youjo Senki crossover that starts literal centuries before canon. There have been some omakes that place during or after canon but they are only about 10, I think.
Depends on the situation and way its being used at least when it comes to writing or story telling.
IRL I would say near unforgivable unless you're forced but in writing I would say it would depend on the character personality and overall outlook on life as well as the current circumstances...