(Not actually new, but new to me.)
I swapped out the spinner HD it came with for an SSD. I made sure to reconnect the fan cable... but I didn't thoroughly double check the connection or confirm the fan was running afterwards.
Either I didn't seat the connector correctly, or more likely it got pulled up on one side when I was positioning and screwing in the fan.
After cloning the new SSD with a drive from one of my other machines, I went to insert a USB thumb drive for an OS upgrade... boy was that thing hot. I happened to have SMC Fan Control on the clone and fired it up. 0 RPM and the temp was 90C and rising because mdstores was pushing the CPU like crazy.
I immediately shut it down and fixed my mistake. I should have known better, but this isn't the first time I have done a drive swap in a mini and didn't mess up before.