Real "UFOs"

Lenticular Clouds

Other natural celestial phenomena
 

'Hole in the sky' formed by melting of ice crystals
(possibly caused by jet plane exhausts)

 

A roll cloud, formed in front of advancing cold fronts
 

"Real" UFO caught on film by Apollo 16 on the way back to Earth:

www.nasa.gov/vision/space/travelinginspace/no_ufo.html

The Autokinetic Effect