Patty stops by unannounced all the time. A few of my other villagers do it too, (especially Chrissy), but Patty is the worst. Startles me sometimes as well.
Typically I'll chat long enough for them to give me the gift, then take a seat. This seems to make them decide to leave. Of course, I could just exit my house as well, but then I'd have to go back in lol.
Lopez always drops by unannounced, but hey, at least I get free stuff out of it. It's just a sign that you and that villager have a high friendship level.
I got my first visitor last night. Interestingly, it wasn't one of my original villagers (although two of those have moved out already anyway) but Graham instead.
I have had a few villagers do "surprise" visits most of them haven't been that bad. I had a few minutes to spare so I started to play just to get rid of items and clear up some space (it was sometime after 3am) then of all of sudden i hear someone saying "are you home"? I was a bit irritated because i was really concentrating on making some space and getting rid of items. Luckily the visit was very short a was given a " milk cart/case" then he left.