I remember one story of a DM who created a library of chaos for his campaign, based on a single word. There were months of visits, ending in a battle against a leather-bound paper dragon that breathed ink. When they killed the dragon, a demon revealed himself with a chuckle, saying “I see you’ve defeated my bookwyrm”.

I not great with estimating sizes, and i often have trouble converting things from feet to meters on the spot. Last session i presented the players a quest to slay a sea monster. They cleverly decided to scout first with a familiar, and i described the creature and its size. I ended up exaggerating the size i bit too much and they’ve decided to avoid it until they’re higher level. So what was supposed to be a simple “monster of the week” type of encounter has now turned into a late game boss fight.

One time I decided to name an enemy faction based on a bad pun. I told the players the name was chosen intentionally, and one of them especially was doing some moderate research to try to figure it out. When I finally played the pun a year later, everyone groaned. It wasn’t even a good pun.

See also hobgoblin mafia existing solely to be referred to as the hob-gob mob. And when players are sent to infiltrate them, it’s called the hob-gob mob job.

If you lean into it you can break people’s brains. “Here’s the key for their fat-cat leader’s ramshackle safehouse, so you can plant this explosive clay in his stove. It’s the Hob-Gob mob job’s hovel hob blob fob.”

I DMed a group that may have had an unexpected nighttime encounter with the bane of all adventurers: the dire rhea.