I don't think it's necessarily like that. The error could make the theorem wrong in all cases.
I think it's more that the mathematician has a top-down way of reasoning, where they can see things like "I want to get from New York City to Los Angeles, so I have to board the bus, take a flight, and then take the bus from the airport at LA". There are certain parts where you basically know that a proof will be possible, because it seems true, like "I can get to LA's airport with public transit", so usually a specific hiccup, like a bus being delayed, won't prevent you from getting there.
Right, it's possible to get to LA's airport with public transit, but the amount of time it takes cannot be upper bounded using currently known techniques.
I think it's more that the mathematician has a top-down way of reasoning, where they can see things like "I want to get from New York City to Los Angeles, so I have to board the bus, take a flight, and then take the bus from the airport at LA". There are certain parts where you basically know that a proof will be possible, because it seems true, like "I can get to LA's airport with public transit", so usually a specific hiccup, like a bus being delayed, won't prevent you from getting there.