Double-tap

Why it fails

A terminal designed for a single conductor clamps evenly on it. Put two in and the screw bears properly on one while the other is held loosely, or both are held at an angle with less contact area than either would have alone.

Loose contact means resistance, resistance means heat, and heat means the connection loosens further. It is the same progression as any other poor termination, arriving at a warm or arcing breaker terminal.

Where it comes from

Almost always from a panel with no spare slots. Someone needed to add a circuit, there was nowhere to put it, and an existing breaker had a screw that would physically accept a second wire.

That is why finding several of them is a signal about the panel rather than about the individual connections: the house outgrew its panel and this was the improvisation.

A few breakers do allow two

Some breakers are specifically listed for two conductors and have a terminal designed for it. Whether a given breaker qualifies is stated by its manufacturer, and it is not something to infer from whether the wires fit.

So "there are two wires under that screw" is a finding to check rather than an automatic fault — but the default assumption should be that it is wrong, because most breakers are not listed for it.

The proper fix

Give the second circuit its own breaker. Where the panel is genuinely full, that means a tandem breaker if the panel accepts them, a subpanel, or a panel upgrade — which is the conversation the double-tap was avoiding in the first place.