using QT 4.1.2, running the following piece of code causes X to crash:

#include <QApplication>
#include <QGLWidget>

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QWidget window;

	QGLWidget *gl1 = new QGLWidget(&window);

	QGLWidget *gl2 = new QGLWidget((QWidget*)NULL);
	gl2->makeCurrent();
	gl2->setParent(&window);
	
	window.show();

	gl1->makeCurrent();

	return 0;
}

X crashes on the last makeCurrent() call.
Constructing the second QGLWidget with &window instead of NULL prevents the crash.
running on:
QT 4.1.2, using a slightly modified code that compiles with QT 3.3.5 doesn't
seem to crash.
Xorg 7.0.0
Fedora Core 5 (2.6.16-1.2111_FC5)
on VMware