What is a Ruling you Regret or wish you would have handled differently?

There are probably more than I can count, honestly. Rules-as-Written only cover so much and it is difficult to come up with a new one on the spot. Fortunately so far I hadn't had much of a backlash because of my rulings. There was maybe some discussion afterwards, but it was still amicable and mostly curious in nature. All in all everybody at the table had a good time. And that is what counts in the end.

I follow a rough outline when creating a new ruling. First is the intention of the player. It is a good question to just ask them, what exactly they try to achieve. This might be obvious but often it is not. From that, I try to establish potential risks associated with the complete action or interaction and discuss these with the player. If multiple players are involved, then the fallout will affect all of them. Then I let them roll if needed.

What is also important that during a session zero, is to establish with the players, that my ruling is final. This reduces table talk and arguments. I will often also talk to the players afterwards, if I researched the interaction a bit more on the internet and might change future rulings according to that new information. This is more for actions and interactions that might come up more than once (for example multiple skills of characters interacting with each other).