More an annoyance then a problem as apps start and run fine. Not sure when it started but would like to not have to see them. It's like there's a verbose switch set to on.

QPainter::begin: Paint device returned engine == 0, type: 2
QPainter::setCompositionMode: Painter not active
QPainter::begin: Paint device returned engine == 0, type: 2

running KDE 4.7.2 & qt libqt4-4.7.4

Old message, annoyance is still present...
in my app, it is on any creation of a Plasma::slider

m_slider = new Plasma::Slider();

QPainter::begin: Paint device returned engine == 0, type: 2
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted

Unfortunately those messages are printed by Qt itself directly, and cannot be silenced to the best of my knowledge. However, they do not indicate a serious problem, and can otherwise be ignored.

However, they do indicate a bug within the Plasma::Slider class, which may be justification for a bug report at bugs.kde.org.