I hit my gas meter, though it was just the clear plastic sight-cover I knocked off. Not so ironically a few days prior, the gas company had come through and replaced the whole meter portion and the new plastic sight-cover was an inch deeper, so my usual strategy of how close to mow was thwarted while I was watching the gas line so as to not hit it with the deck, and proceeded to hit the sight-cover with the wheel instead.
Back when I mowed with a bush hog or bat wing, I used to hit rocks in a field all the time but it just meant I better be watching closer if I don't want to have to sharpen and rebalance the blades soon.
Never really had any mower damage from obstacles except once when I went onto a curb of a concrete slab too fast, it broke a steering knuckle weld... took longer to get the axle off and prep/clean it than the 2 minutes to weld together.
I also, do not think it is possible to damage an engine from a deck blade impact through a belt drive, unless the engine was already in really bad shape and soon to self destruct anyway.