Given the possibilities of an alien spacecraft observed on Earth violating the known laws of physics, or some error on the part of the observer, I'm going with the latter every time.
First, prove that everything we know about physics is wrong at a fundamental, irreconcilable level. Then explain why our completely wrong models of physics still work as well as they do. Then explain the Fermi Paradox in light of the apparent existence of easy faster than light/antigravity technology and confirmation of the existence of other technologically advanced civilizations in the universe. Then I'll be willing to concede the still practically nil chance of any of those aliens actually being here given the vast size of the observable universe as being likely enough to consider.
Don't get me wrong, I want it to be true. I desperately want it to be true. I've been fascinated by UFOlogy and sightings and the related folklore for decades. I want some fate for humanity other than us slowly choking to death on our own poison, alone on this island in the midst of vast seas of infinity. It's just that the bar for proving any other possibility is higher than a third-hand account of someone seeing a light in the sky that moved really fast.
