Mallus said:
While there might have been shared mistaken interpretations of rules, most of the house-ruling went on as a part of a deliberate attempt to customize the game to suit different individual tastes.
It hasn't been in my experience. I moved to Australia for a year, which was my first real foray into playing for a long time with a group other than the people I know here.
Since I was thinking about moving there permanently, I joined a couple of home games of people I met at local gaming stores. While there I played in Living Greyhawk games under about 10 different DMs and played in 5 different DMs home games. I also discussed rules with people regularly as they walked into the gaming store I hung around in.
In almost every case of a "house" rule in all that time, it was due to someone who didn't know the real rules.
For instance, when I mentioned to someone that they should be a rogue since it was a high level game, it was good to have a lot of attacks, they should get a weapon of speed so they could get the most sneak attacks per round. One of the DMs in the area overheard and said "That's SOO dumb, you are such a rules lawyer, you can't make more than 1 sneak attack per round" When I attempted to explain to them that the rules in the book didn't have that limitation, they said I was lying. Then they looked it up and realized they'd been playing with a house rule for 3 years without knowing it. Then immediately declared the rules were stupid and they wouldn't let anyone have more than 1 sneak attack per round. Rogues were supposed to be sneaky, not do lots of damage per round.
I ran into someone who was POSITIVE that critical failures were part of the rules. According to him "They've been that way since 2nd Edition". When I told him that according to core 2nd Ed rules, they weren't part of the rules either, he told me I didn't know what I was talking about.
This is fairly common amongst DMs I ran into. Don't know the rules, don't care. Once tried to tell a DM that neutralize poison didn't clear all of the ability damage ever done to poison to us. (I was at 2 con, and I should have kept my mouth shut, but I'm fair that way) He told me to stop interrupting him and questioning his decisions, it was HIS game and he KNEW what the spell did. So, I got cured of all con damage and shrugged.
Now, you may not have encountered this if you mostly play with people who are the type to frequent enworld. However, especially on conventions when I play Living Greyhawk, where house rules aren't allowed, I've run into at least 1 fairly big house rule per DM I play with. All of whom thought they were following the RAW.