I have a problem with products with no image. I have set the Product Image Placeholders in the backend at
System > Configurations > Catalog > Product Image Placeholders
It works well with the majority of products with no image.
But often it happens that instead showing the placeholder, Magento shows a random product's image.
Tracking the problem I realised that the catalog media cache has this wrong photo, even when the backend shows that the product has no photo at all. This is an example path:
I cleared all the caches and the wrong image is regenerated again. The only solution I could find is: select the product in the backend, and without changing anything, hit the "save" button.
I hope I explained myself clearly.