r/DnD

This probably is more of an issue with online games, but twice this weekend I've been in games that used critical fumbles (1 on an attack roll means something additionally bad happens) without listing or stating that as a house rule. In the first one, I was blindsided by "you shoot an ally" mid-combat, so in the second one I asked the DM if they use critical fumbles when I didn't see any house rules listed. I was told "yeah, but it's not important."

Critical fumbles are a particularly egregious example of this; there are good reasons why the DMG recommends not adding additional affects to attack rolls of 1, including that it disproportionately disadvantages classes (primarily martials) who get multiple attacks. However, I've seen other major changes to how things work—"You can't cast a cantrip as an action and a bonus action leveled spell in the same turn [in 5.5e]," nerfs to sneak attack, etc.—imposed as house rules mid-game with the justification that they are "rule of cool" or "what the DM says goes." I suspect that in some cases the real reason is that DMs don't know the basic rules very well and just use something they read on some site or think a house rule they played with when they started actually is RAW.

It is true that the DM is the arbiter of what the game's rules are and that players can chose not to play if they don't like the rules. However, if the DM never communicates those house rules, then the players can't choose not to play beforehand. I think it is best to communicate house rules up-front so that people can decide not to join or to bow out before the game starts.