The short URL links they give you when you click the share (arrow) icon on a photo or album seemed to break sometime Monday night/Tuesday morning (UK time). I mailed an album link through to someone on Monday evening and it worked fine, passed it to someone at work on Tuesday morning and it didn't work, neither did any of my other albums. The Flickr support forum had a bunch of threads on the subject. Haven't looked at it since, but at the time if you copied and pasted the link from your browser address bar then that would work, not sure about photos. I presume photos shared from there to forum threads would be broken at that time too.