Holding the Surface Pro with one hand while drawing with a Surface Pen on the other is apparently enough to "break" the tablet. A small but annoying bug reduces the accuracy of the Surface Pen if the user is touching the screen at two or more spots simultaneously. Microsoft is offering no time table for when users can expect a fix.
https://www.notebookcheck.net/Surface-Pro-owners-growing-irate-over-annoying-Surface-Pen-bug.290226.0.html