Apple Photos on Mac OS 10.11 (El. Capitan). Some photos have mixed up thumbnails. Basically this means that preview thumbnail is one photo, but when a user double clicks on the thumbnail, a completely different photo is displayed…

Apple’s old iPhoto used to have an option to re-built thumbnails. Unfortunately the new Photos app does not currently have this option. If the thumbnails are completely messed up, it’s probably easiest to delete the whole library and re-download from iCloud. If only limited number of photos are affected you can force thumbnail refresh by rotating the affected photos clockwise and then back anti-clockwise. This can be done for multiple photos at once.

Mac OS 10.11 (El. Capitan)
Apple Photos
12.2015