The same thing is happening - the return code being passed to my delegate (from [NSApplication endSheet:returnCode]) isn't being treated as the NSInteger it ought to be. At least I think so. Here's the trace, and you can see that the value 1(NSOKButton) is being treated as id, causing problems (0/NSCancelButton doesn't crash as it's a special case) :

Program received signal: “EXC_BAD_ACCESS”.

(gdb) bt

#0 rb_ocid_to_rval [inlined] () at /Users/parzival/devo/buried/MacRuby/trunk/include/ruby/ruby.h:1252

#1 0x0000000100130161 in rb_vm_ocval_to_rval (ocval=0x1) at compiler.cpp:6155

#2 0x000000010110537c in ?? ()

#3 0x00007fff820c29f9 in -[NSApplication endSheet:returnCode:] ()

#4 0x00007fff81fd523e in -[NSApplication sendAction:to:from:] ()

#5 0x00007fff81fd519d in -[NSControl sendAction:to:] ()

#6 0x00007fff8206068b in -[NSCell trackMouse:inRect:ofView:untilMouseUp:] ()

#7 0x00007fff820911a3 in -[NSButtonCell trackMouse:inRect:ofView:untilMouseUp:] ()

#8 0x00007fff8205f135 in -[NSControl mouseDown:] ()

#9 0x00007fff81f79967 in -[NSWindow sendEvent:] ()

#10 0x00007fff81eaf122 in -[NSApplication sendEvent:] ()

#11 0x00007fff81e45acc in -[NSApplication run] ()

#12 0x00007fff81e3e798 in NSApplicationMain ()

Attached are a few files as a test case, an NSWindowController subclass and MainMenu.nib that can be dropped into a new project to check this if anyone wants to. I recall when I first had this problem that I could see what the change was, but unfortunately I've forgotten that now.