Rob and Geraldo, thank you for sharing your experiences.
I raise the issue to Canon more than 10 weeks ago. Big saga. Until now the problem is not yet resolved. Basically samples have been taken my the local technical guys, and escalated to the South East Asia regional department, who then said they do not have the expertise to deal with it and then passed it on to Canon Japan. The regional technician's reply FROM CANON INC JAPAN was that the printer was behaving normally, and this is the printer's limitation. It will mark thick or uneven paper. Screw them, it marked the thin smooth Canon semigloss paper which they claim is their 'standard' paper for testing. The regional tech guys did eventually come to personally check on the printer and he remarked that it was very bad, he had never seen anything like it. The recommendation was to change the rollers. 1.5 weeks later they brought old, used, dirty rollers salvaged from their demo printer that's a few years old, and tried to pressure me to let them install it in my brand new less than 6 month old printer. Finally I wrung their hand for a 1-1 replacement. The local tech is insisting that all printers that use rollers to pinch the paper for feeding will surely mark the paper. I was also told Canon may not even want me as a customer anymore and might refund me my money if the replacement iPF8400 has the same problem because it is not fit for my usage...%$*@!