D&D 5E What are some Rules misunderstandings that turned into houserules in your game?

[Flamestrike]

It's more of a old rune than misunderstanding:

We roll initiative after 1st round(surprise round),

My personal bugbear rule. No offence, but that one is not for me at all.

 

[amethal]

Aha! So more an old rule than a house rule.

You can also make an Acrobatics check in Pathfinder to reduce falling damage, although it only allows you to treat the fall as 10 feet shorter; presumably Tumble did the same in 3rd edition D&D.

As I understand it, there's nothing to stop a 5th edition DM allowing something similar, since the "skills" are not really defined any more, so I wouldn't even call it a houserule.

 

[Oldtimer]

There's some evidence linear falling damage in D&D was originally supposed to be quadratic (as makes more sense from the physics point of view).

No, it doesn't. The kinetic energy at impact is directly proportional to the initial height (disregarding air resistance).

To answer the question in the OP, when we started with OD&D, we didn't realise that a magic-user has limited spell slots. We assumed that if you knew Magic Missile, you could cast it every turn. We played it like that for years.

 

[Voadam]

To answer the question in the OP, when we started with OD&D, we didn't realise that a magic-user has limited spell slots. We assumed that if you knew Magic Missile, you could cast it every turn. We played it like that for years.

I always disliked the resource management aspects and thought this would have been a great way to go if spells had not been so powerful in damage compared to weapons. It would also mean that a flavor spell would be more likely to be prepared and used if it did not take up a precious limited combat resource.

 

[jmartkdr2]

Nat 1's and 20's being auto-fails/passes on saving throws. It's swingier, but some people really like that and facing a save you can't possibly pass isn't fun.

The edge cases where it works against players (casters heavily invested in concentration) are specific enough to get specific exceptions (I usually recommend it as an added feature of Warcaster - the damage needs to exceed your con save bonus to require a save at all.)