From Canon:

Thank you for using Canon products.

We have learned that some users of the Canon EOS 5D Mark II digital SLR camera have identified two types of image quality phenomena that appear under certain shooting conditions.
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Notice from Sigma:

Sigma’s EF-530 DG SUPER EO-ETTL II, EF-530 DG ST EO-ETTL II, EM-140 DG EO-ETTL II, EF-500 DG SUPER EO-ETTL II and EF-500 DG ST EO-ETTL II do not operate roperly with Canon EOS 50D D-SLR camera. To overcome this, we will start supplying a free-upgrade service to our customers today.
Please notice that this problem occurs with the EOS 50D D-SLR camera only.

We deeply apologize for any inconvenience this may have caused to our customers.

Upgrade will be required to overcome the following problems

1) The Autozoom function does not work properly.
2) Exposure is not correct.

More information

From Sigma:

SIGMA APO TELE CONVERTER 1.4x EX DG for Pentax mount and APO TELE CONVERTER 2x EX DG for Pentax mount are not compatible with lenses epuipped with HSM. Therefore, these Tele Converters cannot be used with the APO 50-150mm F2.8 II EX DC HSM for Pentax mount and APO 70-200mm F2.8 II EX DG MACRO HSM for Pentax mount.
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