I put a nail into a wall and hit a cable — the lights flicker when I move it. What should I do right now?

To be concrete about the sequence:

1. Do not touch the nail or anything attached to it.
2. Go to the panel and switch off the breaker for that circuit. If you cannot identify which one, switch off the main.
3. Confirm the circuit is dead — the lights that flickered should not come on with their switch.
4. Put a note on the breaker so it stays off.
5. Leave the nail where it is and call an electrician. Removing it does not help and disturbing it can make the damage worse or leave a live conductor exposed inside the wall.

If the circuit that went dark also feeds something you need — a fridge, a medical device — run an extension from a different circuit rather than switching it back on.

For next time — and this is worth saying because it is entirely preventable — a cable and pipe detector costs very little and finds this before it happens.

Also worth knowing that cables commonly run vertically above and below outlets and switches, and horizontally near ceiling level, because that is where installers put them. Treating those zones as no-go for fixings avoids most strikes even without a detector.

Turn off the breaker for that circuit now, and leave it off until it is repaired. That is the whole answer and everything else is detail.

What you have described is not a minor fault. The lights changing as you move the nail means the nail is carrying current, or is intermittently making and breaking a live connection. Both are serious:

• The nail is energised. It is metal, it is in a wall, and it may be touching plaster, a fixing, or whatever you were hanging. Anyone who contacts it while grounded is at real risk.
• The cable is damaged and thinner where it was struck. Current squeezed through a reduced cross-section produces heat, inside a wall cavity, next to timber and insulation.
• An intermittent connection arcs. Arcing is how fires start in walls, and the fact that the state changes as you move the nail means it is arcing every time it shifts.

Do not pull the nail out with the circuit live. Switch it off at the panel first, and label the breaker so nobody helpfully turns it back on.

On "is it acceptable to leave it until someone can come" — with the breaker off, yes, that is exactly the right state to be in. With the breaker on, no, and I would not sleep in the house that way.

The repair itself is usually straightforward for an electrician: open the wall at that point, cut out the damaged section, and join it properly in an accessible junction box, or replace the run. It is a small job and a bad one to improvise, mostly because the joint has to remain accessible and correctly enclosed.

One useful thing you can do meanwhile: photograph the location and measure from two fixed points. Once the nail is out, finding the exact spot again in a plastered wall is annoying.