[ard0r]

I entered some random URLs I had open, hit the button, and got an alert "Something is definitely wrong" when I entered a URL with several parameters: ?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+SlickdealsnetFP+%28SlickDeals.net+Frontpage%29

I removed this bit and it worked fine.

Did you already implement jk86's unique URLs suggestion? It looks like you did!

[onur]

It gives the warning when thumbnail generation failed for some urls and less than 2 urls left for displaying. Actually I did not implement it yet just watching the real time visitors from GA =)