Any flash plugin will cause an immediate galeon exit from an X11 error :

> galeon --sync --disable-crash-dialog http://www.youtube.com/
The program 'galeon' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadMatch (invalid parameter attributes)'.
  (Details: serial 120 error_code 8 request_code 146 minor_code 3)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.) 


I talked to Philip Langdale, but he had no immediate suggestions. Since the
flash plugin works fine with both Firefox and Epiphany, I suspect something is
wrong either with the way galeon is build (against firefox-devel), or possibly
some environment problem makes the plugin unhappy...