I've always been fascinated by three aspects of UFOs:
1) They appear to ignore relativity speed and mass limits - unless you come up with a way to avoid passing through space. I love the idea of warping via Black Holes - can't help feel the gravitional tide stretching the crew and ship out like spaghetti ought to be a bit of a drawback though....
2) So many of them crash land when they get here -what sort of advanced civilisation can build a dinky little spaceship to cross light years of space, but has yet to master the concept 'landing gear'?
3) Why the little guys who crash their ships promptly set up the equivalent of a Fiat Punto production line in Nevada for the US, rather than doing a quick repair job and RTB/Conquering the planet/becoming zillionaires by drip feeding stunning new shiny gizzits via web shops.
(You might also wonder how, if the situation were reversed, the average aircrew would make out trying to build, say, a TriStar from memory).
Dave