The only thing that comes to mind here is that you must "Disable Stretch" on all audio tracks you import for mastering because they will likely have embedded tempo data if exported from Reason - this will cause them to play back at the wrong tempo, unless it just happens to match the tempo of your mastering song file. :-)
Giles Reaves, aka 'selig'
Audio Illusionist, Musical Technologist
Selig Audio, LLC