J.W.
Unless you have a solid connection, which is actually quite rare arcing can occur. That is what happened. The outlet needs to be replaced end of discussion.
Since you felt the need to mention it, whether the humidifier was on is irrelevant.
Since I am a know it all a humidifier should be on a 220. It doesn't have to be but it should. :) Okay I don't actually know it all, I think it is a 220 but I am not at home to look at our extra outlets and I am not bothering my husband with the question. In other words a standard outlet will arc because of the voltage needed for a humidifier.
I was raised by a pack of carpenters/electricians, I called them my uncles. :p