Reproducible with the following example on Windows - dragging the widget out from the bottom makes it disappear, and even releasing the mousebutton does not stop the dragging (it is still possible to move it back into the docks):

#include <QtGui>

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

QMainWindow mw;
mw.setCentralWidget(new QTextEdit);
QDockWidget dw;
mw.addDockWidget(Qt::BottomDockWidgetArea, &dw);

mw.show();
return app.exec();
}