A couple of possibilities
If you are using physical response, you might be having this issue (this frame does not draw, because pr corrects the positions before screen output, but this is what would be happening in that split second).
- figure1
Note too that position is determined not always by the strongest, but by ANY angle involved.
- figure2
So in figure2, if you have a round actor, you're almost always touching two sides at once if you touch it at all. This can be unpredictable sometimes, as 'stair stepped' pixels create differences that you can't see normally.