There have always been rumors about a hidden message in "Another One
Bites the Dust".  What will you *really* hear if you play it backwards?
Probably whatever you want to hear.  It has been rumored that this song
says 'Its fun to smoke marijuana' when played backwards.  The band has
always denied this, and there is no real reason to believe otherwise.
Most people who have listened to it think that you have to have
the phrase in mind and use your imagination to hear it.  Even more
convincing, several people also tried recording the words "another one 
bites the dust" in their own voices and then playing them backwards.  
They decided that those words just coincidentally sound vaguely like 
that particular phrase since their own voices sounded much the same way
Queen's did when played backwards.

There are places where music or words are recorded backwards in Queen's
music, but none with a "hidden message".  

The beginning of 'Ogre Battle' is actually the same as the end of the
song, but played backwards.  The "stairstep" guitar at the end of Teo 
Torriatte was actually recorded in descending order and recorded
backwards to produce the ascending stairstep sound.  In "A Kind of
Magic" the word "magic" is recorded backwards a few times.