The initializer list of the offending object contains a call to QGraphicsRectItem(x, y, width, height) where x and y are scene coordinates. Is this the wrong way to do it? The doc for QGraphicsRectItem is a little vague about this.

The only other place where I modify the item's rectangle is when the user wants to change the rectangle's width and the height is changed proportionally:
this->setRect(this->rect().x(), this->rect().y(), width, width/WH_RATIO);