I got a tingle from a wet saw (tile project) the other day, and was curious as to why the GFI didn't trip. I punched the test button and sure enough nothing happened. So my question is, do I just replace the recpticle with a new one and be done with it, or do I worry about what caused it to go bad in the first place? It is an outdoor recepticle on a detached garage with a leaky roof, so I worry about it getting wet. But there is no obvious signs of water in the electrical box, and presumably the outlets say "test monthly" (I wonder how many people do, but I'll being doing it more often from now on) for a reason. The outlet is about 8 years old. So how common is this?
Pretty common. Especially for units installed outside. The elements getting in, however little, do have an effect.
Just replace it and don't worry. Do test them from time to time as you said.
Today's GFIs are MUCH better than 10 years ago as well.
Another thing I would do is install an "in-use" cover. This will keep the weather out even when things are plugged in.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2011 NEC.