Perhaps the grenade has a little dial on the top you turn for various power levels of the propeller, or something. Come on, it's sci-fi! Almost anything is possible! It might be silly, but damn, I would've suffered without grenades in TFTD. Flares too. Night missions are impossible without flares, or tons of phosphor torpedoes.
even better!

Maybe a mega flood. OMG you found Noah's sub.

One thing that could be considered "unrealism", but is probably more akin to the world being internally inconsistent.

Any RPG that has the following:
1. Resurrection spells available to the player
2. A character who gets killed as part of the plot, with no character even trying to use resurrection magic on that character.

There is one famous example I'm thinking of, but it's not the only one.

Thank god he turned the TP roll around. Didn't think he was ever going to fix that.

Holy god... the human race needs a patch.

I can usually overlook weird near-impossibilities like the sub thing, personally. The thing that always gets me is character stupidity; my suspension of disbelief ends where characters begin to act brain damaged. "Oh, the villains are after X thing? Let's get it first and keep it in a much less safe location!" Or as happens in one of the recent Fire Emblem games: "Mwahahaha! I'm so evil that I'm going to abandon the self-restraint I practiced in other games and instead begin to ramble endlessly about destroying the entire world! Surely that won't backfire terribly and drive everyone to the main character's cause because the writers couldn't come up with a more natural way of having everyone team up against me! Mwahaha again!"

So much rage.

Or maybe it's just you that needs to install that "sense of humor patch", since you're suffering a bug where you can't recognize a joke.

Hmm, the only thing I can think of are silly bugs that are common to happen, most often physics bugs, ragdolls freaking out constantly is a sign of poor QA.

Or I suppose in Oblivion it pissed me off how the leveling in that game worked, took me out of the experience.

Both.

Artificial boundaries in an open world.
These could be invisible walls, but not only. Any construction which blocks the way without a reason that applies in the world's logic, falls into this category

In Oblivion, for instance, levitating over the mountains around the map is impossible without an in-game explanation. There is a reason in reality. It's impossible for the player to explore more than the developers have built, but it's out of the game's world.

I found it extremely stupid and enjoyment-breaking when I tried to play Amnesia and it seemed that a big part of the game was lighting candles and torches, and yet I could not take a torch off a wall or a candle out of a candle-holder to light more candles/torches and I had to rely on finding "tinderboxes", and then every single "tinderbox" only had 1 use (which makes it not a tinderbox at all, but a solitary match.)