Edited from Sigma Japan’s announcement:
Sigma’s lenses for Sony mount may have a potential aperture operation problem when used with the Sony α33 and α55 Interchangeable Lens digital cameras.
To overcome this issue, we will be offering, free of charge, a modification service to our customers who have purchased a Sony α33 and α55 and . . . → Read More: Sigma lens incompatibility with A33 and A55