This reminded me of an evil prank I did on some friends of mine when in university - CD burners had just become a thing, and at the local concert venue where we volunteered, a handful of burned CDs soon appeared at the mixing console with various music the engineers liked to listen to while getting ready for a gig.

Anyway, I ripped the discs, added a nice 50Hz hum under the music and burned new copies which I then left by the CD player.

Yup. Cue frustrated sound engineers trying to debug the ground loop which only manifested itself when the CD was playing.

I never dared admit to the prank, but rather swapped the hum CDs for the originals before someone got a chance to investigate this thoroughly...