(From GIT master branch on 30-Nov-2012)
I was working on a project with multiple mixer lines, then clicked the "New project" icon. It seems as though the mixer still had some references to the lines, while the QT objects had been cleaned out when the repaint request came in.
The backtrace is:
#0 0x0815de94 in QVector::size (this=0x95abd18)
#1 0x0815c88b in FxMixer::channelSendModel (this=0x84b7898,
fromChannel=120 'x', toChannel=0 '\000')
#2 0x081c5b3a in FxLine::paintEvent (this=0x88dae08)
#3 0xb766c062 in QWidget::event(QEvent) () from /usr/lib/qt4/libQtGui.so.4
#4 0xb76128ac in QApplicationPrivate::notify_helper(QObject, QEvent) ()
#5 0xb7617659 in QApplication::notify(QObject, QEvent*) ()
This was a bug in the old master branch (with the new FX mixer). It's fixed in stable-0.4-new-fx-mixer already.