Visual astronomy is a very subjective matter because the human brain can easily be fooled. I'll be the first to state that my sketches are merely an impression of things that I think I've seen, without claiming any true scientific value.
A popular example of such optical illusions is this bright planetary nebula in Cygnus, which bears the nickname "Blinking Planetary". The reason for this is that many people report that it seems to fade and reappear very quickly, as if someone's playing games with a light switch. Obviously this is wrong as planetary nebulae generally do not change brightness overnight, let alone in a matter of seconds. The illusion's caused by its very bright central star. When focusing on it with your eye, the surrounding - much fainter - nebula seems to disappear somewhat. When you then turn your gaze away from the star, the nebula reappears. No magic, just our eyes having difficulties adapting to different brightnesses when already observing under unusual (very dark) circumstances.
What's more interesting though, are the bright patches at the nebula's border, on either side of the central star. Scientists call them "FLIERs" (Fast Low-Ionisation Emission Regions), the origins of which are still not well understood. One theory goes that they're gas that was hurled out from the star about a thousand years ago at supersonic speed, but in that case their bow-shock points in the wrong direction. Another theory suggests that these patches of gas are stationary and that the expanding gas bubble scrapes past them.