https://www.youtube.com/watch?v=LPdYH1ffFi0

I just saw this video. Some lights that are on, occasionally flicker when most lights are off. But when all lights are on, nothing flickers!
What is causing this?

Voltage drop on that circuit?

Possibly a loose neutral connection.

This is a warning from his electrical installation that he needs to have his fire insurance in order.

Depending on the specific electrical topology it can indeed be a loose neutral. If it's 120-0-120v 180 degrees shifted, everything on the lighting circuit will essentially be able to run without a neutral if all lighting circuits are equal in power. They're effectively run in series on 240v. When the system's unbalanced, one of the branches will take (for instance) 160v while the other only gets 80v. That is of course very bad for any device running from those circuits.

When you completely switch off one of the 2 120v circuits, the remaining circuit needs the neutral and if that thing is slowly burning out a wire nut connection or something like it, there will be no return path for the current and the lights will flicker until the wire nut has welded itself semi-together again, until it heats up a bit and breaks its connection again etc.

Thank you for the responses! But the video seems to be now deleted...