IE Problems
I've added addthis to a bunch of our sites we manage. For some reason under IE7 we get a Operation Aborted javascript error causing the browser to show an error page saying cannot load page, the relevant info I can find about this issue is here: http://support.microsoft.com/kb/927917.
The other issue which I think has only just started recently is under IE8 the entire pages content disappears. A script error symbol briefly appears in the status bar but then disappears so can't see what the error is.
Here's a page where we use it, although for now addthis is not being rendered in IE7 or IE8. If you save the page in IE9 and open in IE8 or IE7 you can reproduce both issues.
http://www.sportspro.co.nz/products/1052-mens/7021-scott_250_black_boots.aspx