I've been working on this project, where the main crux of it is that Facebook users register as members of the site (with one-click registration enabled). As I've been testing, I've been unable to produce any errors.
However, as we're very very close to launching it, the clients have been bug-testing it, registering themselves on the site. For the most part it's fine, apart from Safari. Three separate people on different machines have all reported the same thing; when they click to register, Facebook's little 3-bars loading animation appears, and then nothing happens. When I check in the back end, all the people who reported a problem are not registered as users of the site, which suggests that it's Facebook that's never reporting back.
Being on a PC, I had to borrow a friend's Mac to test it, and it worked perfectly for me (typical), whether I was logging in or registering as a new user. I also tried it on Safari on my iPhone, and it also worked perfectly.
The only thing I noticed in a screenshot from one of the testers is that one of Facebook's JS files was perpetually pending in its response - specifically 5p0cp47osg-.js from ak-static.fbcdn.net - but I don't know how crucial this is to the login process.
Is this likely to just be a particular security setting that these people just have their browsers set to? They also have no problems in any other browser.