If you have a house with steel rain gutters, you'll want to check them for rusted patches. Catch the problem early and it's a cinch to fix. Just buff off the rust, clean off the area and coat the hole with plastic cement. If a large hole is already there, you can treat it by laying down aluminum foil or plastic on top of the plastic cement. If it's too big even for this method, consider either replacing the section of gutter or laying in a piece of metal flashing to bridge the gap.