Amateur astronomers spend a lot of time looking at the sky, and are pretty knowlegable about what they see there. They've all seen flying things they can't identify--UFOs. These are UFOs I've seen that might have been alien interstellar spacecraft, but probably weren't.

1) A half-dozen bright flashing strobe-like lights in the sky, over a period of about half an hour, on two subsequent nights. The flashes were like a bright star, and appeared to be in the same spot.
Best Possible Explanation: Solar Reflections off a geostationary satellite.

2) Two tiny, rectangular objects transiting the face of the sun's disc, about 20 seconds apart, while observing the latter with a solar filter on a small telescope.
BPE: Spacecraft maneuvering to dock.

3) A faint, round, transparent cloud roughly the size of the full moon, moving about the same speed as a satellite, seen through binoculars. The cloud was expanding, and became dimmer as I watched for several seconds until it disappeared altogether.
BPE: Orbital spacecraft venting fuel.

4) An hard, sharp echo on a surface-search radar tuned to not detect aircraft (a late 60s analog unit, so it wasn't a digital glitch), traveling about 3000 miles an hour, and approaching with a few miles of my ship. There were other witnesses, and I have heard similar reports from other sailors. There was no sonic boom.
BPE: Malfunction, or a radar jamming signal used by either our forces, or the enemy.

5) Brief, bright, naked eye, clouds at very high altitude at night, quickly dispersing.
BPE: Deliberate dispersals of wind-tracking flourescent dyes at high altitudes, or venting of fluids, by spacecraft or high-flying aircraft.

6) A pulsing, metallic, amoeba-like body shining with an orange color, seen through a small telescope at the very limit of visibility and observed for several minutes as it moved across the sky.
BPE: A tied-together cluster of aluminized mylar party balloons reflecting sodium vapor streetlights.

7) A sharp-edged, rectangular building, or bridge-like structure, of approx aspect ratio 1:4 (otherwise featureless), seen spanning a crater on the lunar terminator.
BPE: A chance illumination and cast shadow effect.