I noticed a slight fizzling sound when I turned on the landing light.
So, I undid the switch and had a look inside.

First of all, I should say it was a double switch. One side was for two
way switching to the floor above, and the other side was three way
switching for this floor. That's an awful lot of wires to cram into a
back box, even quite a deep one.

Anyway, it turned out that two of the conductors were completely loose.
They pulled out as soon as I eased the switch away from the wall. Also,
the earth wire was loose, although it stayed in its terminal.

This switch was fitted in 1998, and AFAIK it had not been touched since.
It has worked fine since then, and it's working now since I tightened it
up. So, what's going on?

My guess is that it's all so congested with all those cables that it was
never properly tightened in the first place. But, how come it's worked
okay for 20 years?