I regret not going into business for myself when I had the chance. My body regrets every minute spent with a tool in hand.

Wasn’t this a movie? Dude, Where my car??

Steel tank? Shut off valve on the tank outlet and leave it.

This is interesting and as a mechanic for the last 28 years I am going to give your friend some back up. I could not even come close to telling you how many times I have replaced failed open thermostats. I can tell you in that time span I have replaced less than 5 failed close and specifically only remember doing exactly 1. So for them to have never seen it is very possible. I will ask a few other techs today that will give decades possibly a century or two of experience and I would bet the total will be very low.

That said, I am not going to completely discard the comments from this who say they have seen many and is very common. The reason for that is this failure could be brand (vehicle or thermostat) specific. A brand that I do not use or see in the field often. I do recall seeing something about a Chrysler product that had a know stuck closed issue. I will ask today and let you know how often others I know have seen this. Should be entertaining.