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My first guess is that your fax machine is possessed by a demon. My second guess is your machine needs to be cleaned. From HowStuffWorks:
"For dark and light pages, it could be as simple as changing the darkness or density setting on the receiving machine. If the receiver does a test print and the page comes out clean, then the problem is most likely with the sender's machine.
"The sender should open up the document feed area of the fax machine and clean all the surfaces and moving parts with a slightly damp cloth. If the fax machine is one of those all-in-one gadgets that doubles as a scanner and copier, clean off the scanner glass as well with a little glass cleaner.
"If the test print comes out with lines and splotches, then the problem is on the receiving end."