At our GenCon panel on horror, we got asked about the risk of breaking atmosphere in Trail of Cthulhu games by asking for Stability tests. You describe whatever horrific or disturbing sight the investigator encounters in ghastly detail – and then go “now, roll Stability”, dragging the player out of the story and soiling everything with bald mechanics. I don’t entirely agree with the premise – sometimes, switching to mechanics at a moment of high tension lends huge dramatic weight to the roll – but if it resonates with you, then what you need is a bad die.
A bad die is a die that’s dedicated to a particular purpose. Ideally, it’s visually distinctive – I’ve got a d6 with skulls for pips that gets designated a bad die in some games. The bad die is only used for one type of roll only. For example, in a Trail game, it might only be used for Stability tests. If the GM hands the bad die to a player, the player knows it’s time to make a Stability test, and that failure would be costly. There’s no need to say anything in the heat of play – the GM makes it clear before the game that if you’re given the bad die, you’ve got to make a Stability test and that failure will mean a big Stability loss.
You can use bad dice for other purposes. You could have a bad die for Sense Trouble rolls, or Heat checks in Night’s Black Agents. In 13th Age, you might designate a particular d20 as the bad die for Last Gasp saves. As long as the bad die can be easily distinguished from other dice, and the players are told beforehand what the bad die entails, it gives the GM another non-verbal channel to communicate with the players.