[DMing] What was your most embarrassing screwup as a DM?

Switching from 3.5 to 5e I messed up big time.

I was coming back to DnD from a long hiatus after 4e came out.

My first screw up was not realizing that the amount of xp it takes to level is vastly different between the two systems. On top of that I forgot you were supposed to divide the xp by the total number of party members. Fortunately I caught this early on, gave them some temporary hit points for a bit, then we had a leveling session next game and got everything back on track.

The other huge mistake I made was not realizing the shift away from magic items in 5e. I gave the party a bunch of homebrew magic items based on the 3.5 DMG. I ended up keeping two of those items and switching out other gear for stuff in the DMG. Fortunately everyone was really understanding and was pretty happy about the change.

Now we've been plaything these campaigns for about 6 months and having a blast.

In one sessions my PC's were investigating a sinking temple for some valuable item/NPC/info. And i was building up to a climax with a BBEG, blocking several exits with the rising water, rumbling getting louder etc. And i couldn't find the stats of this BBEG in my prep... i literally said; "Ah well, it was never here then..." "For real dude?" "yeah i can't find it anywhere man, sorry"

That was the end of the game night and we all went home feeling unsatisfied...

In one encounter, I forgot to give the bulk of the enemy mages a turn. Oops. Their leaders got a turn, but not the others... So these magical badasses all got slaughtered, and I didn't figure it out until afterward.

Too many....

That my players never know about because DM's never let them see it, just weave it into the story.

The worst i've done is make an NPC say something completely stupid that derailed the plot for the players...it was later found out that the NPC was actually a doppelgänger and now the players found new info off the doppelgänger's corpse.

I, in a conversation between the resident Cleric and a Gnome Artificer Shopkeeper, kept replying to OOC comments by other players in the background without dropping accent, spun it into the Gnome hearing voices in his head, laughing madly, and bidding the Cleric goodbye so he could go 'bash the voices out' with a warhammer.

My players were protecting an NPC and helping him get on the last airship out of a dangerous city at night. Two PC's rolled high enough to notice a distant figure climbing onto the airship right before it took off, into the cargo hold through a different entrance. However, I accidentally described it:

"You see the assassin climbing into the cargo hold just as the airship takes off."

The PC's looked at me and went, "Wait, ASSASSIN?"

Yep, definitely did not mean to say that. Oh well, they still had fun tracking down the assassin though!

For the first session, I made up really nice wanted posters that were set to lead to a quest line no matter which direction the PCs headed in. One was for a Goblin bandit captain. I forgot to set them out while the PCs were at the tavern.

As planned, a caravan guard who barely survived an ambush down the road staggers in, begging someone to rescue his employer from the goblins that attacked them.

Cue: the first dungeon. Players get two rooms in, rescue the Caravan leader, start the room he's in on fire, and book it to the first major city, never seeing the rest of the dungeon, grabbing any of the loot I had made cards for, or learning about the goblins' master or his plans until the city is being flooded with goblins pouring out of the sewers.

I'm sure, eventually, I'll find a use for the dungeon and get them a chance to resolve that optional other quest line. It hasn't happened yet, but nothing goes to waste when you're DM-ing. Still, hard not to kick yourself about a simple mistake having such resounding consequences.

I replaced their lowest dice when stat rolling with 6s. So the half dragon had like 24/26 str. Luckily mythweavers shit the bed and world exploded

forgetting to take my meds :P
it was an interesting game...

Letting my players snort modron dust.

I once arrived naked and noone said anything until we had played the entire session.

Or not :P