PCWorld: Overclock at your own risk: Intel axes its overclocking warranty.
https://www.pcworld.com/article/3609834/overclock-at-your-own-risk-intel-axes-its-overclocking-warranty.html

Jim Bell's note:

I over-clocked a Z-80B (6 MHz spec) to 12 MHz in 1978, cooling it with a thermoelectric device.