The shorter program posted by Chris crashes, too. (I am using Qt version 4.6.3 on Linux).

The backtrace suggests that l.show(); causes the crash, but there are no parameters hinting at the problem.

Here is the backtrace:
Qt Code: 
  1. 0	QTextLine::layout_helper(int)	/usr/lib/libQtGui.so.4	0	0xb77f1fae	
  2. 1	QTextLine::setLineWidth(double)	/usr/lib/libQtGui.so.4	0	0xb77f3dbc	
  3. 2	??	/usr/lib/libQtGui.so.4	0	0xb76d8c0a	
  4. 3	??	/usr/lib/libQtGui.so.4	0	0xb76d99c1	
  5. 4	QFontMetrics::boundingRect(QRect const&, int, QString const&, int, int*) const	/usr/lib/libQtGui.so.4	0	0xb77c3f3b	
  6. 5	??	/usr/lib/libQtGui.so.4	0	0xb79c5fd1	
  7. 6	QLabel::minimumSizeHint() const	/usr/lib/libQtGui.so.4	0	0xb79c6416	
  8. 7	QLabel::sizeHint() const	/usr/lib/libQtGui.so.4	0	0xb79c653b	
  9. 8	QWidgetPrivate::adjustedSize() const	/usr/lib/libQtGui.so.4	0	0xb75bddd9	
  10. 9	QWidget::adjustSize()	/usr/lib/libQtGui.so.4	0	0xb75c4c3d	
  11. 10	QWidget::setVisible(bool)	/usr/lib/libQtGui.so.4	0	0xb75cdb80	
  12. 11	QWidget::show	qwidget.h	485	0x08048992	
  13. 12	main	main.cpp	9	0x080488ad
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Same in the program above: w.show() leads to the segfault.

It seems that this behavior was already detected as a bug in version 4.6.3 and fixed in 4.7.1
(Check http://bugreports.qt.nokia.com/browse/QTBUG-13546 and http://bugreports.qt.nokia.com/browse/QTBUG-11427)

In fact I should have found this earlier on my own, so thank you for helping me nevertheless.