The other time was on final approach to an airfield in a Twin Comanche when my wife yelled lookout and I saw the wheels of a Piper Arrow appear in my windscreen. I dived and banked away and went around for another go.
I was coming in on the normal approach slope and he had gone around the circuit and came in over the top of me without seeing me as I was below his line of sight. He was above me so I didn't see him either.
It didn't help that there were three training aircraft also in the circuit which was only air ground and when I called 2 ahead final to land, he though he was the second and I was behind him.
Which is why it's a good idea to include the type and possibly even color of your a/c when approaching a non-towered field. Doesn't necessarily mean the stude in the circuit knows what a Twin Comanche looks like, but he may understand that it's probably not the PA28 in front.