Or putting your darkroom under the laundry room, when the washing machine fails and dumps water all over your darkroom. Then you try to clean it up and leave 50 rolls of developed 120 negatives neatly stacked on top of each other (not in sleeves) from your once in a lifetime trip to Venice, Italy. All is fine after the leak, until you realize that days later, the high humidity from the leak has caused all of the developed negatives to stick together. All ruined.
I now sleeve and file immediately. Wish I learned that one earlier.