An agent/subject of the SCP Foundation has gotten his hands on a [DATA: THE EXPUNGING] card that makes card games a serious business: Said card can destroy the other player... or its user, if he loses.
There's also SCP-514, a flock of doves that disables all weaponry in its area of effect and nullifies violent tendencies in everyone around it. This results in teams from rival organizations who are trying to take control of the SCP to use....alternate methods of conflict resolution. Like rock-paper-scissors, riddles, or card games. This has led to the Foundation's commanding officers commenting on how off-putting it is to see two grown men in full tactical gear taking a children's card game so seriously.
In Dominic Fear's Kenny Bassender (Full Title: Kenny Bassender's Quest For Greatness With the Underground Association of Puppydog Racers) movie, Kenny Bassender is a normal person who isn't special. Until he starts playing a game called Puppdog Races, where he is the flawless. So great, that the other members of the Association try to kill him. Not the whole society (it still is in normal present day America), but very serious.
On LiveJournal, roleplay is very serious business, as evidenced by the "Roleplay Secrets" community, a daily post of nasty things anonymous roleplayers have to say about other roleplayers, allowed to rag on anything from their characterizations to the size of their avatars. Similar is the "RP Anon Meme", a bi-monthly explosion of hateful anonymous discussion. People have actually made death threats over pretendy funtime games on the internet.
A meme on the Japanese Internet (a translated version of which is well-travelled on anonymous text boards) involves a somewhat lengthy rant about visits to Yoshinoya being really serious business. Just read it. Variations are popular for ranting about extremely petty things. The meme itself was actually referenced on Sayonara, Zetsubou-Sensei, that's how widespread it is.
This (subtitled) episode of the Québecquois series "Tom et ses chums" ("Tom and his pals") has the titular character playing a game of D&D with old friends. What he doesn't realize beforehand is that they've kept playing every week during the years he hasn't seen them (meaning they have absurdly high-level characters), and for them (barring the GM), this is very serious business.
When they demand that his new character start at level 1 (making him useless since the encounters are tailored for a high-level group), and then belittle him for being a peasant, he gives up and decides to be The Loonie.
