I've used many fudges and tweaks and mods to games over the years but I don't think I've ever actually sat down and said "This is a house rule."
I think the 1 thing I tend to incorporate into all of the games I'm running is that measure of "Does that make sense?" to any odd situations.
Now we're all playing in unreal worlds, even if that world is a mirror image of our own it's still experienced through the suspension of disbelief. So by their very nature the games are going to have things happen that in the real world wouldn't make sense but can be explained through the setting itself.
However, there are still some pretty easy checks that are likely universal.
e.g. If your character falls off a 500ft cliff, with no means of rescue and has no super power or attribute that would suggest otherwise, the chances are your character is dead.
Similarly if the characters are all in an underground chamber and there's an earthquake causing the roll to fall in then chances are that they're dead.
The thing is I also tend to contradict this "house rule" when it comes to death although that's the easiest situation to explain it.
I don't like having characters die in a game unless it suits the story that the group wants to tell. That includes player characters and non-player characters. Yes that does mean I'll wave away poor dice rolls at times. Don't get me wrong if the characters decide that they're going to take on a horde of dragons with nothing but pitchforks then obviously that's just lunacy and they expect to die.
So my accompanying house rule is that character death should never be without purpose, even if that purpose is temporary insanity on the part of the (player) character...
Similarly I have other GM traits that don't really constitute as house rules -
- If neither the GM nor the players can remember the specifics of a rule then make something up that fits the moment rather than breaking the cycle of play to check the rulebooks. In some cases these rules become the norm in others it's a case of checking what the actual rule is in-between sessions.
- Dice rolls only count if they've landed on the table.
- When playing in a campaign if a player cannot attend then that character fades to the background or become Passive with no negative impact on their development. Well, within limitations anyway.
Think those are the only house rules that I apply in general but none of them are specific mechanics that I always apply.