Flamestrike
Legend
As an example, the second campaign I've ever run in 5e, the party came across a goblin village with high walls. When I wrote the encounter 2 months prior, I expected the PC's to have to negotiate with the goblins, if they made the goblins mad, they'd pelt them from the high walls with their shortbows. I suspected it would be quite the incentive to negotiate and not just attack. Well, I let a player play an Aarockra and it turned out quite different as they didn't even say a word to the goblins as he clobbered them a bit and opened the gate from the inside, effectively turning what I balanced into a one-sided beatdown.
See my tower example above.
You (the DM) failed to take into account the abilities of your players characters. They didnt trivialise your encounter; you did when you designed the encounter and failed to account for a PC ability.