Some SO profile pages won't load at all, others will load but you can't go to the Activity tab, others are absolutely fine.
- Me: Profile fails for me, seems to work for others (including me if I use a different browser and don't log in) | rep tab fails
- Tholle: Profile works | rep tab works
- gnat: Profile works | rep tab fails
- Jon Skeet: Profile works | rep tab fails
Presumably the fact my main profile fails for me but not others has to do with me being signed in as the person whose profile I'm viewing.
(Why did I choose those accounts? I chose my account because, well, that's where I first hit the problem. I chose Tholle at random. I chose gnat because he'd had a similar problem before. Finally, I chose Jon because he's a long-term, high-rep user with a lot of activity.)