Buzz Aldrin came forward a few years ago to say he witnessed a UFO during his Apollo 11 mission. Now, you can say he just saw space junk. The problem is that he knows all about space junk too and further, he's had 40 years to reevaluate his opinion on what he saw. Not only did his opinion not change, but he took it a step further and came out publicly, at risk to his own reputation, to say he witnessed a UFO.
Gordon Cooper, another retired astronaut, went on the record to say he had two encounters with UFOs. The first was as a pilot in the Air Force while flying over Europe in 1951. The incident has never been officially explained. He and his fellow pilots witnessed objects flying in formation faster and at altitudes higher than their planes were capable of reaching. Six years later, while overseeing testing at Edwards Air Force Base, a flying saucer landed and was reportedly filmed. After viewing the footage, Cooper sent it ahead to Washington and nothing else was ever heard of the incident. He also talks about airline pilot friends who witnessed UFOs at relatively close range.
To me, these are two expert witnesses who know the difference between a weather balloon and something else. Its accounts like Cooper's and Aldrin's that make it difficult for me to be skeptical on the subject of UFOs.