If I call QThread::quit() or QThread::exit() (which is pretty much same) before actual running of an instance of QThread it will spoil the message loop forcing it to exit immediately. It seems wrong because exit() as a regular member function must not affect the thread in the not running state and quit() as a regular slot must not keep/remember an incoming signal if there's no message loop in the current object.
It's easy to reproduce:

MyThread thread;
thread.exit(555); // or quit()
thread.start();
...
MyThread::run()
{
  QThread::exec(); // exits immediately with exit code 555 or 0 in case of quit()
}

Full example is attached.