It is possible within an exclusive QButtonGroup managed list of buttons to end up with multiple buttons appearing in the toggled state. By rapidly clicking buttons on the widgets in an adhoc manner, the user occasionally ends up with more than one button appearing in the toggled
state. This has been verified a couple of times in Qt 4.3 and 4.4.0, and only when using the CDE style.

Test case main.cpp to reproduce:
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#include <QtGui>

class Buttons : public QWidget
{
public:
Buttons() : QWidget( 0 )
{
setGeometry( 50, 50, 300, 300 );
QGridLayout * l = new QGridLayout();
QButtonGroup * gp = new QButtonGroup();
gp->setExclusive( true );
for ( int i = 0; i < 100; i++ )

{ QPushButton * b = new QPushButton( QString().number( i+1 ), this ); QSizePolicy p( b->sizePolicy() ); p.setVerticalPolicy( QSizePolicy::MinimumExpanding ); b->setSizePolicy( p ); b->setCheckable( true ); gp->addButton( b ); l->addWidget( b, i/10, i%10 ); }

setLayout( l );
}
};

int main( int argc, char * argv[] )
{
QApplication * theApp = new QApplication( argc, argv );
Buttons * b = new Buttons();
b->setVisible( true );
return theApp->exec();
}
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