A client of mine has a Canon imageCLASS D1120 copier that would jam each time they tried to copy or print any double sided. I looked and looked for a jam, but couldn’t find any. It would print the one side and suck it back in and jam. I took a flashlight in the back and finally figured out what was happening — there was paper inside that was causing the jam.
Remove the paper tray and look for this green lever (as is shown in the diagram #1 in the picture below). Pull it down and remove the jam!