What's The Worst Feeling In The Game?

This is a broad topic, so I'm curious. People talk about things that are "feels bad" a lot, from things that are meta and unfun to things that are counter, yet important to a character concept or build. And sometimes, it's even just being a Hasted Monk and missing 5 attacks or having your big Spell fizzle because the enemy made their save. And further still, it's rolling 2 Nat 1s with Advantage.

So yeah, I'm curious to know. What's the thing that's felt the worst for you either experienced or seen or what mechanic makes you cringe, that the designers swore was amazing and you just weren't feeling it?

For me, I actually got to watch a Hasted Monk whiff five attacks. And last night, that player (that happened years ago) playing a Rogue, (who was also hasted his next turn) whiff with Advantage and roll 2 nat 1s.