Very strange screen anomaly

Today I bumped into a very strange screen anomaly. I'm not sure StrokeIt is the culprit but it is surely affected. Whenever I do a gesture the screen darkens and I can see the traces of some previous strokes. After releasing RMB the screen returns to normal and the command assigned to the gesture is executed. You can see it here:

[url=http://img248.imageshack.us/i/clipboardimage.png/]