When a flying machine is going down, the top observer will glitch and disappear for 2 ticks (0.2 sec) before reappearing. when the flying machine is going up, the bottom observer will glitch momentarily. 

how to recreate

1: build a vertical flying machine

2: make it go up

3: look at the bottom observer

it should be glitching 

4: turn the machine around and make it go down

5: watch the top observer, or stand on it

it should be glitching too

I have a video with a 50x slow motion of the glitch.