When much younger, I was in a physics class at QIT. My mate and I were using a CRO to monitor an electrical circuit. We had to twiddle knob in defined increments and write down what happened to frequency, as a resistance changed - something like that.
No matter how we twiddled (you lot are cheeky!) the waveform never changed. So we called over the tutor, and he twiddled too (I've told you once!). He checked the circuit, then went away scratching his head.
Further twiddling by me on my own (Stop laughing!) revealed that the wave form stayed the same frequency, but a tiny little bit of fuzz on the beautiful sine wave changed - the fuzz was much less than 5% of the amplitude.
The tutor came back, looked more closely at the CRO, glared at us then rotated a knob on the CRO. Bingo! The waveform now did what it was supposed to do.
You see - the couple of feet of wiring in the circuit was picking up about 40 volts of mains hum form the huge main sub station transformer on the other side of the room!
And one of us two idiots had set the CRO to the wrong time base setting.... :-)
Now I don't know about harmful, but I can honesty say that the experience left me with a permanent effec effec effec effec effec effec effec effec effec effec effec effec