What was your biggest told-you-so moment?

For me, Ghosts of Saltmarsh, start of campaign. We were fighting four sahaugin, and the wizard cast Thunderclap, before bonus action casting Shadow Blade. I attempted to tell the wizard’s player to wait until next turn to cast Shadow Blade, but they cast it anyways.

The sahaugin then attacked the wizard, breaking concentration on Shadow Blade. I hated being right. Not that it mattered in the long run, as we defeated all the sahaugins.

What was your big I-told-you-so moment in DnD?

In my homebrew campaign a the party was inspecting house where a meteor struck and a couple party members checked the basement while the rogue stayed on the main floor and a monster crawled out of the meteor and attacked the ones in the basement.  On his turn the rogue decided to jump down the hole to tackle the monster.  I told him that because of the angle he is standing at, he would need to roll a nat 20 in order to land the hit.  He doesn’t, and in addition to the falling damage he took additional damage from landing on the meteor.  As a consultation prize I allowed him to roll behind the meteor and use it for sneak attack 

Quote from bthec18 >>

In my homebrew campaign a the party was inspecting house where a meteor struck and a couple party members checked the basement while the rogue stayed on the main floor and a monster crawled out of the meteor and attacked the ones in the basement.  On his turn the rogue decided to jump down the hole to tackle the monster.  I told him that because of the angle he is standing at, he would need to roll a nat 20 in order to land the hit.  He doesn’t, and in addition to the falling damage he took additional damage from landing on the meteor.  As a consultation prize I allowed him to roll behind the meteor and use it for sneak attack 

 And did the sneak attack work?

It did

I had a game I was DMing who got lost in a maze of sorts. The party went left and ran into a group of guards but didn't alert them and were outnumbered. Then returned to where they came from and one of the players rolls a Wisdom check for a hint at which direction they should go. There were only two directions to go: right or left, and so I respond, "You believe going right would lead you away from the patrol.", then with that information he convinced the party to go back left where they run into the roaming patrol of guards and get taken prisoner.

The party broke out of the prison later because there is always a way out but that player was all upset about it and I was like come on man you gotta be kidding me I told you go right and this was like right after you all returned from going left. How much more clear could ut have been. To be fair we were all drinking and other stuff. So, they were at the junction for like an hour joking around.