The reMarkable tablet’s file format contains an angle field for each point, which is the angle of movement, in radians clockwise from the positive x axis, range [0, 2π). For most tools (BallpointPen, CalligraphyPen, Marker, Pencil, MechanicalPencil), it’s the tangent from the last point, but for a few tools (Fineliner, Paintbrush, Highlighter), it’s the average of the last two line segments’ angles—not the tangent from two points ago to the current point, but the average of the last two segments’ tangents. The consequence of this is that if you’re drawing a line with one of these tools in the positive x axis direction, you’ll regularly end up with points with spurious angles, as the average of just above 0 and just below 2π is around π.
(There’s another field speed, calculated in pixels per sample, which also has this curious average-of-last-one-or-two-segments behaviour, so that I can synthesise both of these properties perfectly for all except the first point or two of every stroke, provided the stroke hasn’t been scaled or rotated. I also feel like grumbling about the fact that marker tilt is not recorded in the file, though it influences the width field in the CalligraphyPen and Pencil tools, which you might otherwise expect to be derived from some combination of pressure, tilt and speed.)
This is just a fun curiosity I observed when reverse-engineering it all (and I was really pleased that I predicted it as soon as I discovered the average-of-last-two behaviour, and that experimentation proved it to be so); I haven’t gone far enough to identify the effect of the angle property on rendering, but Fineliner and Highlighter certainly won’t use it (I suspect Paintbrush does).
My first two guesses on this Boeing issue are: some sort of mishandling of the point of angle discontinuity, or division by zero.