Groan.
OK, you select a 'random' encounter for your PCs instead of rolling, and throw it at them. A bunch of mindless Zombies shamble into the room, as a portcullis drops, trapping the PCs in the room. The encounter is supposed to be a medium difficulty encounter, to spice up an otherwise quiet session. The campaign has been going really well, the players have all gelled, are working together well as a team, and have bought into your campaign. You all have great chemistry and the last campaign ended with a disappointing TPK, which was nearly the end of the group.
Your dice are on fire. You roll natural 20 after natural 20. One by one the PCs drop, and they continue to roll poorly. The mood at the table is grim. The Paladin stands grimly over the bodies of his downed comrades, down to 1 HP, but he has a smite remaining, and is facing an equally badly wounded Zombie (the last monster standing). He also has enough lay on hands remaining to stabilise the rest of the party.
You go before him, roll an attack behind your screen, and it's a natural 20. If the result stands, it's a certain TPK, in an encounter not connected to the story, and through no fault of the players.
Do you TPK the party, or fudge the roll?