My hot water heater is in the bathroom. One of the previous owners had built a box to cover the hot water heater by covering a wooden frame with beadboard paneling. I use this box as a table and keep books, lotion, blow dryer, etc on it.
Anyway, I moved the box to clean under it and noticed that the coupling on the hot water outlet was covered in rust, and a pool of water had collected in the depression. Needless to say, there was a leak in the fitting.
Image is to show the depression where water pooled. This is not my water heater.
(This was one of the times that I was glad that I'm picky about cleaning under things, otherwise we wouldn't have known about the leak until it was pouring out onto the floor).
After watching it for a couple of minutes I was able to determine that the water was coming up over the compression nut and running down to the depression and pooling. So, I figured that the compression ring has deteriorated and failed.
The rust looked bad and I was concerned that it may have eaten into the water outlet fitting. I was worried that we would have to buy another hot water heater. So, I wasted a week doing research, and asking my maintenance friends a bunch of questions.
Finally I worked up the nerve to actually try to remove the compression coupling and see how far the rust went. It turns out the rust was only on the surface and I was able to repair the leak by cutting the copper pipe back a few inches, and replacing the compression ring and nut.