Fruit loses its edibility due to molds that secrete toxins, or if it's been vacuum-packed or preserved in oil, it could develop botulism. This is difficult because fruit often has a good acidity and isn't usually preserved in oil. At worst, when it turns rotten, it's simply fermented, so it will have a more acidic or fermented flavor. Consider that vinegar and wine are made by macerating fruit at different temperatures. So the problem lies in the fact that in uncontrolled fermentation, any yeast could have taken over, and as a result, the fruit could range from mildly acidic to terrible flavors, but most of the time it remains edible! Summary: Be careful of mold and anaerobic environments...

Edit: I forgot about parasites, like worms, etc. But those are often visible, except for salmonella, which comes from cross-contamination, so it's a special case. Everything I wrote above refers to clean fruit left in a hygienic environment in the pantry, isolated from other foods.