The compositor crashes Blender on user input when it cannot find referenced files on the hard disk. (i.e. ctrl-shift click on the image node to connect a different socket to the viewer node in the attached file.) This is also a problem when the frame range values are off and therefore are pointing at non existing images.

System info:
Windows 7 64bit, GTX570 and GTX580, Blender 2.63 b50364