If you activate an observer by updating him, then start moving the observer while it still gives out power, and then reload the game before the observer finishes moving, then the rotation of the observer will change.
If the output of the observer was facing into the +y direction, he will rotate to face into +z.
If he was facing into +z then he´ll face into +x.
If he was facing into +x then he´ll face into +y.
Same happens with negative directions. -y goes to -z. -z goes to -x. -x goes to -y.

Here´s a video demonstrating the bug:
https://www.youtube.com/watch?v=qFKHRlapufQ