drawing errors

[Dp0H]

I've prepared the project:
qtguierror.zip
Press the buttons to animate
That is awful..

[wysota]

I don't have time to test everything right now but from skeeming your code I can say your setRect() implementation is somewhat incorrect (it shouldn't influence the primary problem though).

setPos() should be expressed in the coordinate space of the parent item, so putting it in the same call as setting bounding rect for the item might be confusing for the user.

As for the main problem I'm guessing it all have to do with inability to express every fractional number in the datatype used. Increase your scaling and all should be fine.