A common source of damage to photo prints, and negatives comes from handing and mishandling. Damage can come from oil from fingerprints, dust, and improper handling. Mold can be a problem even in dry climates, when prints have been improperly handled. Touching film or photos with your fingers will transfer fingerprints and oils to the surface, which encourages mold growth. Photographic cotton gloves are best for handling film and photos when they are dry. When wet, use latex gloves.
Writing, whether in marker or pen, on the back of a photo can cause unexpected damage. The act of writing on the back of a photo can leave indentations in the image. Ball point pens, crayons, and acidic markers cause various kinds of damage over time when chemicals from the writing implement transfer to the image causing discoloration or weak spots.
You can often see the bleed-through on the other side of the photo. Additionally, if a photo with ink on the back gets wet, the ink can run and bleed onto other images, or soak though to the image side of the print.