Tried thermal paste on a CPU that was already 6 years old. Bad idea.
I had this old Dell Optiplex from 2018 that was running hot, so I figured I'd re-paste it with some Arctic MX-4 I had lying around. Pulled the heatsink off and the old paste was basically dust, but I didn't clean the die properly and got a tiny grain of rice stuck under there. Booted up fine for 2 days, then the temps spiked to 95C and it shut off during a BIOS update. Had to order a whole replacement motherboard off eBay for $40. Anyone else wreck a system trying to do a simple maintenance job?