I've been known to rewind 35mm halfway through a roll and change to a different film. I just mark the frame number on the leader with a marker.
I've tried to run 120 all the way through the camera and then wind it back onto another spool for reuse. When I tried it with once with Neopan 400 it actually worked. In false confidence I tried the same thing later and epic failed. Has anyone ever successfully done this or am I just being too big of a cheapskate?