The other day, at university we were mounting a circuit for test. When switched on, it didn't work (nothing surprising yet).
After looking for what was wrong for a while, I measured the DC power supply (configured for simmetric +- 10 V) with the oscilloscope between GND and the positive terminal (and also between GND and the negative terminal).
For both cases I got a 20 Vpp sinusoid at 50 Hz (line / utility / mains frequency). I measured the power supply disconnected from the rest of the citcuit with the same result.
Another scary thing is that the multimeter shown +10 V and -10 V (it should be 10/sqrt(2) V for both terminals).
The workaround was to move to another bench with another equipment and continue the exercise, but I'm curious about what can cause this effect.