Mold and Mildew
Mold and mildew are the biggest risk your slide film faces today, especially Kodachrome film. Slides stored in damp climates are very susceptible to this damage. The mold will actually eat away the chemical emulsion on the film and there is no way to remove it from the film without leaving its mark on the film. Digital ICE (IR Cleaning) will not remove this type of defect from the scan.
Needless to say, the sooner you have the slides scanned the better -- because this mold and mildew will continue to grow until the slide image is totally destroyed.
The four images below are actual examples of mold and mildew damage to the slide film emulsion.
Each image was cropped from a small area on four different slides.