When the player moves their head upwards, the vertical rotation will occasionally jump.
To reproduce, look straight down and then look up. The player's vertical rotation jumps instantaneously from near 90 degrees (straight down) to between 0 degrees and -90 degrees (straight up).
While the effect does occur at other angles, it is most noticeable if performed as above.
Slow upwards motions when rotating upwards between 0 and -90 seem to not trigger the effect but slow motions upward between 90 and 0 do. Fast motions upward between 0 and -90 seem to make the vertical rotation jump to between 90 and 0 and fast motions upward between 90 and 0 seem to make it jump to between 0 and -90.