Pipe Thread Compund Plugged Control Valve.
An interesting experience that I wanted to share: I discovered a gas leak at the pipe that entered the control valve on a hot water tank. When I disassembled the piping and removed the nipple from the control valve, I noticed that the screen inside the control valve was nearly covered with a yeloow substance. I removed the screen and discovered that it was pipe dope thread sealant that had covered it. I'm surprised the tank was even working as 90% of the screen was covered.
There was no drip leg and very little pipe dope around the threads of the nipple entering the control valve. The tank is about 17 years old.