Our DM uses cards which add a special effect on critical success and critical failure. Some of them are benign, some of them are severe.
There are 4 of us in the group. We walk into the room, and the mind flayer immediately stunlocks 3 of us. My character is a Loxodon Cleric built like a tank, but he can't break the stun because I keep getting shit rolls.
On its next turn, the mind flayer runs over to me. It goes to reach out its tentacle attack which will flay my mind and make me its slave... and crit fails. Crit fails with advantage.
The card that got drawn for the crit fail made it injure its tentacles on my armor. That may not have been a problem, except that they got injured in such a way that they could not be used until the next long rest. Think, like, breaking an arm, or something. But that meant that ALL of the flayer's most dangerous attacks were now unusable.
So we fucking thrashed it in the next turn, because it was completely defenseless. In our world these monsters are extraordinarily rare and most people wouldn't even know they exist- when we brought its body back to the wizard in town he FREAKED OUT and we were just like "yeah this weird tentacle monster was there, it acted like it was a big deal or something but it died when we breathed at it with no problem at all."