When looking at a photo in Apple's Photos.app I noticed a white exclamation mark in the bottom right corner of a photo. What does that indicate?
After tapping around a bit I tapped on Edit and a message popped up that said there was any issue downloading the photo over cellular. I guess it was just a thumbnail. I recently turned on iCloud Photo Library and turned off cellular data for photos. Turning cellular data back on removed the exclamation mark.