Part of gaming is suspending disbelief. Sometimes you need to do it for the world ("There is magic? Okay, I'll accept that.") Sometimes you need to do it where the rules smooth over reality to make it easier to just up and play.
Here are a few from GURPS that work well enough for a game, but which don't match my personal experience of reality. I choose to just not concern myself with their reality, because they work as the abstractions they are. If you dislike these and want to change them, have at it - but for me, suspending my disbelief is enough.
Languages - using the study time rules and languages, learning a language is pretty trivial. You can go from nothing to fluent in a short time, and never have communication issues once you get the fluency. Using the study time rules in GURPS, I'd have learned Japanese fluently twice over by now. But the reality of learning a language as an adult is a long slog of studying, learning crutches to get by, undoing the bad habits those crutches bring, learning new vocabulary, training your mouth to make the correct sounds, etc. - it's rewarding but not fun. Making it a smooth, relatively easy process makes for better gaming.
Arm ST - you can buy up ST just for your arms and improve weapon damage. As a trainer and occasionally a fighter, I can tell you, the strength in your upper body isn't that relevant to striking. It's all from the ground up. But honestly, a mechanism to give you a limited form of strength that affects all upper body actions is fine with me. So is being generous and saying it affects damage.
Full-Damage Dual Attacks - You can attack with two different weapons for full damage. Which is hard, in reality, because of the hip thing. It's possible to leverage your hips into a strike with two weapons, but it's not easy to do so, and it limits what strikes you can do. But again, modifying DWA so it does less damage depending on the type of strikes, your body positioning, etc. isn't as fun as just say "It's -4 to hit plus off-hand penalties" and just moving on. I suppose you could reduce the damage for the strikes in the first place ("they both do -1 or -1 per two dice, whichever is higher") or something, but that's making the weaker ones drag down the stronger ones. It's just as abstract, really, and requires just as much suspension of disbelief.
Linear Learning - Skills just improve in a linear fashion. You get better at the same potential rate until you hit whatever practical limits there are in the game. In reality, you need to keep working at an increasing rate to see any real improvement. But in a game, that discourages you from improving to a degree (something I saw often in Rolemaster), and it's not necessarily fun. A straight-up linear increase system is fine; modulating growth by moduling the value of improvement works fine, too.
I can't always do this. I can't just say "there are dragons and magic, so therefore realism it out the window, so therefore nothing needs to hew to believability." That's one reason I prize the believability of GURPS, and why I like reality checking. But it's a game, and I'm willing to take "close enough" and "get rid of the stuff that isn't fun" for all they can be worth. The things I mentioned above I think are a bit unrealistic, but they don't break my suspension of disbelief in the same way as other things might. What breaks the disbelief for others might be different - it's had to get a game going where all of the players have the same acceptance of all that occurs. The fun of games occurs in that area where there is a mutual suspension of disbelief, in my opinion.