Setting up one evening I had taken the scope out just before dark to line it up with the south while I could see what I was doing. I pulled the cap off and pointed it at a random patch of sky, to check the columnation while I still had light, and put an eye piece in and looked through. Two enormous square objects passed across the field of view
and I thought, "That's it. The aliens have arrived.
It took me a couple of seconds to figure out what it was that I had just seen and then I dived for the laptop to check. The ISS had just passed over head and I had glimpsed it through the eye piece.
Have also had a few small objects slowly cross the field of view and not been sure what I was seeing until I realised they were geostationary satellites.