Thanks for the replys guys. As I would like to understand the bug first I will try the workarounds after that.
After you pointed me to the sorting, I did some more research.
Can someone explain the strange stepping through the code in QtCreator?
(the order of the steps when I use "Step Over" are placed at the end of the code lines)
Qt Code: 
  1. case RS2::EntityCircle:  
  2.   a = center.angleTo(*v);                                                  1.
  3.   dist = reversed?
  4.               fmod(sa - a + 2.*M_PI,2.*M_PI):                              3.
  5.               fmod(a - sa + 2.*M_PI,2.*M_PI);                              2. 4. 6.
  6. //            qDebug("angle centerTostartPoint: %f",sa);
  7. //            qDebug("angle centerToNextIntersection: %f",a);
  8. //            qDebug("distance: %f",dist);
  9.   qDebug("counted the useres: %d",v.use_count());                          5. 7.
  10.   break;
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I would have expected the following order of the stepping:
line2
line5 (until now that's the same what QtCreator does; reversed is false)
line9