Many examples could also be thought of as ordinary, non-insurmountable obstacles combined with Invisible Walls. Used as a last resort when developers either don't care, or can't come up with a Hand Wave.
For whatever reason, your character can't/won't just crouch and move through it. The Impassable Head High Hole - an opening you can't enter because the top of your character's head is an inch too high.The Knee Deep Water of Uncrossability - a body of shallow water which may as well be a Bottomless Pit as far as your ability to ford it is concerned.Especially flagrant when the game doesn't otherwise have falling damage. The Ledge of Instant Death, a type of Gravity Barrier, that looks safe to jump down from, but kills you anyway.The Rough Ground of Unwalkability - an area of rocky or otherwise uneven terrain you can't even step onto.The Endless One Story Staircase - where if you go up (or down) you can climb forever, either because you can't go that way or you missed getting a Plot Coupon that allows you to, and so you could climb those stairs for ten minutes or for three days, but turn around and all you traveled from where you started was maybe a couple meters.
The Gentle Slope of Unclimbability - a slightly inclined piece of land which, despite all logic to the contrary, is completely impassable, both up and down.Is essentially the animation version of I Can't Reach It for climbing characters. Often results in the character making a poor attempt at climbing up it when, on the intended path, they'll throw all their energy into it. The Frictionless Ledge - a particularly lazy method of boxing the player in by simply not allowing the character to jump up to and/or grab a ledge that they would normally be able to.The Frictionless Hill - a slope of arbitrary steepness onto which you can jump only to slide off as if the thing were coated in an industrial lubricant.The Impassable Forest - a sparse congregation of vegetation, apparently sporting a force field that expels player characters.The Unclearable Debris - a pile of rubble of some description that is apparently both solid enough that you can't move any of it, yet unstable enough that the game won't let you even consider climbing over it.The Indestructible Fallen Log - A fallen tree which, despite having nuclear weapons and space-warping magic at your fingertips, you can't even chip.