I can remember similar : on a project requiring extreme frequency stability and type approval, we installed a state-of-the-art Temperature Compensated Crystal Oscillator with a ceramic encapsulation.
We installed it on a PCB with a plastic case around it.
On test, it functioned perfectly until somebody turned on the light in the test chamber and it hopped frequency.
Turns out the plastic case and ceramic TCXO encapsulation was transparent to infrared light from an incandescent bulb.
Biting the hand that feeds IT
