A friend of mine complained he was unable to download some wedding pictures I'd taken. Last summer downloads worked, but as of a few months ago they stopped working.
Just around the time I deleted my Google G+ Profile.
My friend told me the original sharing link worked, but the download button was grayed out. Sure enough, when logged out of Google I couldn't download images. It was easy to see why, image downloading was turned off in my Picasa preferences.
I don't remember turning that off. Given the way I share images, that wouldn't make sense. My best guess is that when I deleted my G+ Profile Google reset my sharing preferences to the default settings. if I'd known I could have fixed it immediately. Instead I've been sending share links out for months that weren't working.
I assume most of the people I sent shares too simply gave up in frustration, or ended up downloading a low res thumbnail.