As a DM/GM I always want to be fair and consistent with the players. So to make sure that happens, I always jot down the rulings I make as we play. This way I can keep my own past rulings straight, and the players feel like they have a consistent and fair interaction with the rules and past precedents. These rulings can also be called house rules if you like, and I think it is always important that your players know what house rules you have come up with for your games. I always try to give my players a list of house rules before we even start playing, but some of them creep up after you start.
So, if you are running a game out there, make sure to keep a list of what makes your game different when it concerns the rules, and your players will greatly appreciate you being organized and fair. Also remember that the players should have some say on your house rules as well and a discussion with them at the start of a campaign about house rules can make sure that everyone is alright with them.