When is a rusty screw more than cosmetically un-pleasing? Answer: When it is inside an active electric panel, securing a wire to the panel breaker.
These screws have become rusted because of high hunidity in the house and especially in the basement where this panel was located.
Rusty screws lose their ability to conduct electricity effectively, and could create an overheated connection.
Therefore it is highly recommended that such screws and/or connections be replaced.
If your electric panel is in the basement, and possibly has an enclosure built around it, a high humidity condition could exist, which could cause this to happen. Check your breaker connections to make sure they are not rusted.