Canon Pixma MX520 Series Online Manual page 584

1310
Cause
The size of paper may not be compatible with automatic duplex printing.
Action
The sizes of media suitable for auto duplex printing are A4 and Letter. Make sure that the size of the paper
loaded in the machine is correct. Pressing the OK button will eject the paper and restart printing from the
front side of the next paper. The reverse side of the ejected sheet will not be printed.