Paint fast with a tablet using autobrush with configuration size=f(pressure). You'll see garbage left on the canvas.
I tried to reproduce it on QPainter canvas. It had occasional small pixel-sized artefacts. I can't reproduce it with OpenGL canvas. Can you attach screenshot? Is this duplicate to https://bugs.kde.org/275829 ?
Yes, it's a duplicate of 275829, and it doesn't happen with opengl (which is logical, since opengl repaints the entire canvas widget every time, which means no spurious junk can stay on screen). I've seen it happen myself as well, but it depends a lot on which brush and preset you choose.
*** This bug has been marked as a duplicate of bug 275829 ***