It happened to me regularly in a long-running game where I was playing a Battlemaster/Thief that specialized in poisons. For every PC that isn't a Thief, applying poison to your weapon requires an Action, whereas a Thief can do it as a Bonus Action. The ability to apply poisons quickly was one of the key abilities that defined my character. However, NPCs who use poison don't require any action at all to apply it to their weapon--they just get free poison damage (or other poison effects) added to all of their attacks. So the class feature that supposedly made me a great poisoner made me worse at using poisons than every other poison-using character in the game world.
In particular, fighting Drow was maddening. They had better poisons than I did, could apply those poisons to their weapons faster (i.e. with no action), and since their poison wasn't actually an item leftover poison couldn't be looted. I couldn't even pick up their poison-coated weapon and use it immediately because the poison damage was part of the Drow statblock--if I used their poisoned weapon it somehow wouldn't do poison damage.
The disparate mechanical treatment of poisons between my poison-using PC and poison-using NPCs was one of the single most frustrating experiences I've ever had in D&D.
I was frustrated enough at the time that I would have seen it as flippant instead of funny.
