D&D is a game where everyone - no matter how good they are, or how much they train, or what dark powers they control - has a 5% chance of hitting themselves in the face.
(Or throwing their weapon across the room, or flailing wildly and stabbing a friend, or tripping over and lying on their back like a turtle, unable to get up.)
I assume everybody else must already know that D&D is a cross between Conan the Barbarian and The Three Stooges, but it came as a big surprise to me. No-one ever mentioned this undercurrent of Vaudeville. It's the one big element of D&D that you can't get from Appendix N: The omnipresent terror of quick death combined with hilarious pratfalls. Watching your fighter slip over and get impaled in a pit of spikes is somehow both horrible and hilarious.
It would be possible to remove the slapstick, with a bit of work. Instead of Stealth Checks, for example, thieves could get better at stealth by slowly going up a table like this:
Silent walking on soft surfaces: grass, dirt
Invisible in complete darkness
Silent running on soft surfaces
|Invisible in moonlit darkness
Silent walking on hard surfaces: Wood
|Invisible in shadows
Silent running on hard surfaces
|Invisible in low-light
Silent walking on loud surfaces: Metal, Gravel
|Invisible in overcast daylight
I understand that 5e has tried to eliminate hilarious failures by giving the fighter a minimum of 3 damage, no matter what he rolled. This stuff is very sensible. In real life you slowly get better, rather than getting a smaller chance of failing. It makes sense that fighters don't hit themselves in the face 5% of the time, it makes sense that professional thieves aren't always coughing and giving away their position, it fits into the source material to make everyone into competent professionals who do not fail.
After making this table, though, I realised I'd never use it. Slapstick isn't something anyone would ever imagine adding to fantasy, it's been quietly ignored in all D&D licensed comics, books and movies, it makes no sense - and it works. The massive infusion of humour and unpredictability it adds is totally vital to RPG's as I play them.