“Rescuing the theory”

It is not inherently wrong to introduce a new explanation after contradictory evidence appears.

A scientist or historian may legitimately discover a new factor because the evidence reveals something previously unknown.

The problem arises when:

a contradiction appears;

an explanation is invented solely to prevent the theory from failing;

the explanation has no evidence of its own;

every subsequent contradiction produces another ad hoc explanation.

So the problem is not the timing alone.

The problem is unsupported auxiliary explanations introduced merely to immunize the theory against contrary evidence.