Strange that it runs fine on Windows... But you might get an unexpected blue screen later today (just kidding)

The label doesn't have QApplication as its parent, it's 0. So I agree that it's not managed by QApplication and it's not deleted twice.
At first sight, I assumed it had something to do with the object tree. My mistake, sorry about the confusion!

The debugger gave me the following backtrace. Still not sure what's happening exactly, but it may explain why it isn't crashing on Windows, because it's clearly inside an X11 class where it segfaults.

Qt Code: 
  1. (gdb) run
  2. Starting program: test_app
  3. [Thread debugging using libthread_db enabled]
  4. [New Thread 0xb646b6d0 (LWP 12758)]
  5.  
  6. Program received signal SIGSEGV, Segmentation fault.
  7. [Switching to Thread 0xb646b6d0 (LWP 12758)]
  8. 0xb70f0f72 in ~QX11WindowSurface (this=0x8190438) at painting/qwindowsurface_x11.cpp:84
  9. 84	        XFreeGC(X11->display, gc);
  10. (gdb) bt
  11. #0  0xb70f0f72 in ~QX11WindowSurface (this=0x8190438) at painting/qwindowsurface_x11.cpp:84
  12. #1  0xb70eaa55 in ~QWidgetBackingStore (this=0x81902e8) at painting/qbackingstore.cpp:858
  13. #2  0xb6f0802a in QWidgetPrivate::deleteExtra (this=0x818e9c0) at kernel/qwidget.cpp:1622
  14. #3  0xb6f0883f in ~QWidgetPrivate (this=0x818e9c0) at kernel/qwidget.cpp:235
  15. #4  0xb730650d in ~QLabelPrivate (this=0x818e9c0) at widgets/qframe_p.h:62
  16. #5  0xb6b06c78 in ~QObject (this=0x80491a0) at ../../include/QtCore/../../src/corelib/tools/qscopedpointer.h:62
  17. #6  0xb6f194dd in ~QWidget (this=0x80491a0) at kernel/qwidget.cpp:1496
  18. #7  0xb72feed1 in ~QFrame (this=0x80491a0) at widgets/qframe.cpp:242
  19. #8  0xb7305c33 in ~QLabel (this=0x80491a0) at widgets/qlabel.cpp:262
  20. #9  0xb6719481 in __run_exit_handlers (status=0, listp=0xb682d324, run_list_atexit=true) at exit.c:78
  21. #10 0xb67194df in *__GI_exit (status=0) at exit.c:100
  22. #11 0xb6700b5d in __libc_start_main (main=0x8048b64 <main>, argc=1, ubp_av=0xbfffd594, init=0x8048db0 <__libc_csu_init>, 
  23.     fini=0x8048da0 <__libc_csu_fini>, rtld_fini=0xb77ee9b0 <_dl_fini>, stack_end=0xbfffd58c) at libc-start.c:254
  24. #12 0x08048ad1 in _start ()
  25. (gdb)
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