When you start gnash with --fullscreen, or when the loaded
movie contains a fullscreen request in first frame,
gnash is unable to exit fullscreen in a sane way, that is the window size you get when getting out of fullscreen has no dimensions, and you have to manually resize it to see anything.

If you need it, a testcase not requiring use of --fullscreen is here: http://foo.keybit.net/~strk/tmp/ObjReg.swf

Remember to look at your rc files for ignorefscommand not being set to true for the testcase to work (no edit->preference way to change it yet!!)