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