Brother MFC-8440 B - Manual (Page 173)

TROUBLESHOOTING AND ROUTINE MAINTENANCE 12 - 12
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The Jam Duplex message appears if you remove a sheet
of paper from the Face Down Output Tray after only one side
has printed.
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If the Paper Adjustment Lever for Duplex Printing is not set
correctly for the paper size, paper jams may occur and the
printing will be out of position on the page. (See To use
Automatic Duplexing for fax, copy and print operations (For
MFC-8640D, MFC-8840D and MFC-8840DN) on page
2-10.)