Last session, we stumbled upon some cultists attempting to summon a powerful creature from the Far Realms via a ritual sacrifice. We were unable to prevent it, and found ourselves staring down a CR 19 Lovecraftian horror! It was more concerned with killing the cultists (at first), and we knew we were intended to run (we're level 8), but were hemmed in by cultist guards blocking our exit route.
The Cleric threw out Banishment and...turns out the monster has a terrible Charisma save. So now it was a battle to protect the Cleric for 10 rounds! Of course, the enemies did everything in their power to damage the Cleric, and they inevitably failed the Con save to keep concentration.
So now we were fighting a battle on two fronts. We looked at each other and decided to go out in a blaze of glory. We threw everything we had at it- the Cleric blocked it's path and took the Dodge action (though it's attack bonus was so high that only unlucky rolls kept him from being hurt, even with a 21 AC!). I threw a Blistering Radiance on top of it, our Ranger fired some poisoned arrows and it actually failed it's save, ending up with the Poisoned condition for a minute (we were surprised it wasn't immune, as most of our foes are), and the Bard kept spamming Dissonant Whispers until they ran out of spell slots.
At some point, the thing retreated to the back of the room due to Dissonant Whispers, and we closed the door and I Arcane Locked it.
Thanks to disadvantage and the boosted DC, it failed break through the door, and started attacking it. We mopped up the other cultists and suddenly...silence.
We healed up a little, then I said the password and we opened the door.
The creature apparently was also not immune to exhaustion, and it had died. Everyone was overjoyed, but I got to thinking on the ride home. We were not supposed to fight this thing. That was obvious.
But we did, and we beat it. Were we lucky to do so? I won't deny it, but it still feels like level 8's taking on a CR 19 should be nigh impossible, but no one even died! So yeah, bounded accuracy does work both ways, and while it worked in our favor, I'm not really sure that it should have.