There were even reports about crashed flying saucers and little alien bodies with perfect teeth stiffly languishing in Air Force freezers. Everything hinges on the matter of evidence. On so important a question as UFOs, the evidence must be airtight. The more we want it to be true, the more careful we have to be. No witness's say-so is good enough.
People make mistakes.
People play practical jokes. People stretch the truth for money, attention or fame. People occasionally misunderstand what they're seeing. People sometimes even see things that aren't there. Just conceivably, a few might be small comets dissipating in the upper air.