You reach a set of two doors. One leads to a fabulous treasure, the other to a room filled with unavoidable death traps. A fiendish riddle engages the entire party's wits for half an hour's tense debate, until finally you tease out the answer that the left-hand door leads to the fabulous treasure.
Stepping through into the treasure room, you're plagued by a nagging feeling of worry as you stride towards the riches heaped within. Realisation hits you just as the flagstone beneath your foot depresses with a small 'click', and you have just barely enough time to glance back at the wall behind you and realise that, in fact, both doors lead into this room, before the whirling blades, acid spouts, poison gas and a dozen other contraptions ruin your day.
...you become the DM and get to devise the next deathtrap.
When the next hapless reader falls to your trap, it will then be that unfortunate soul's chance to take out the next reader.