This reminds me. It must have been 2009ish. I was at Microsoft. Part of the code I was responsible for ran on the Windows build servers. This code was triggering a kernel bug in the Windows registry. The only way I knew how to reproduce it was by building Windows. Because thousands of Windows builds happened per day, maybe 1 of them would hit it every 1-3 days.

The way I got it in a kernel debugger was to constantly run the problematic race condition inside a VM, when it detected that it hadn't hit, it would roll back the VM to a save state of the code already running in progress ... Took an overnight run to hit it on a machine in the office.

All that said, I'd still say it's not very expensive to run a build machine.