The Raw Inspectors try to avoid spewing a bunch of junk into the display if the MIME type is binary. But because of how the assignment happens today, switching sessions can temporarily cause a mixed state where the headers from a new (text) session are evaluated in parallel with an old session's binary body, resulting in a momentary garbage rendering.
This is a performance bug that should be really easy to fix.
https://groups.google.com/forum/#!topic/httpfiddler/MeAjQMMDftc