| If you have Keep Gesture Window On Top* or Don't Hide Gesture Draw Window* selected in Preferences, there's a small chance that while drawing multiple gestures in sequence rapidly that a segment of a gesture can remain on the screen.
This is a consequence of multiple threads for each gesture complete call colliding with each other. If you experience this issue, you can uncheck both of the Preferences above, click Apply, then draw a gesture to refresh the window.
If this happens to you often, then you may have to run with those options unchecked. The purposes of those options are only to improve performance on lower powered systems and prevent certain flickering in Windows XP.