I’ve been adding posts from this blog to stumbleupon for a while.
But suddenly, I noticed that when I try to add a post to stumbleupon, I fill in all the required details, and when I hit the “Recommend this site” button, I then get a blank page.
At first, I thought there was something wrong, so I tried again.
Then I thought something was wrong with stumbleupon, so I decided to leave it until my next post a few days later.
But that also failed.
But I managed to find the cause: Stumbleupon will block you from submitting too many pages from the same domain.
So, self-promotion will only work about 11 or 12 pages, before you get blocked.
Stumbleupon will (rightly) not divulge the exact rules they use to block self-submitters.
But it would have been nice to get a warning message, rather than a blank web page!