Off all the things that can catch fire...
I was not expecting the $10 temp sensor to go all allahu akbar on me.
Not sure what happened. It's been working fine for ~2 years. I added a 2nd GPU and changed one of the rads. PC was on for ~20s to run the pump to fill everything and I saw the smoke.
I'm blaming this 100% on human failure, It probably got damaged while changing things around.
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