I noticed two nail heads in a floor-to-ceiling moulding have rusted.

This is a corner room, NW corner of house, and the nails/moulding involved are on the NW corner.

What has happened?

Probably just humidity in your house. Could be condensation if the moulding is near a kitchen or bath since it's on the cool side of the house.

Just use a good quality primer to cover the stain and touch up paint.

Steve
steve@paintermatch.com
http://www.paintermatch.com