QPainter::drawRectangle (on a QImage) draws wrong lines (diagonal Line) when given coordinates lie outside of the image's size.
E.g.
QImage(114, 49)
p.drawRect(-404, -22, 1014, 93);
results in a line which is drawn from somewhere left bottom to somewhere right top.

see attached program and wrong image.

Note: This does not occur with Qt5.3.0 beta