After upgrade from Magento 1.8.1 to 1.9.1 I got not working add to cart button for configurable products, fatal error:
Fatal error: Maximum function nesting level of '1000' reached, aborting! in /home/igor/vhosts/www/***/magento/lib/Varien/Object.php on line 625
Full exception here.
I found that issue occurred because of this rule:
Fix for this issue located at GitHub Gist.
Would be great to add this fix in next release.