just happened to me again today...Ruby popped in when I was in the other room trying to put my refurbished furniture down...I got a knock and her saying "dude, let me in!" LOL
This just happened to me now while I was replacing a table by the new fireworks one xD Gwen (she is my fav among the villagers I have) came in and even gave me a cool six lamp!
Bella, Deirdre and Molly visit often. Probably because I talk to them the most (so that they don't move). I am expecting Olivia, Muffy and Francine to visit soon since I am speaking to them more now as well.
I have had it happen a few times. It always seems to be after doing some kind of action like adding a new K.K. song to my stereo or picking/placing/moving furniture.
I love it when it happens but one time I was clearing the main room in my house - I wanted to rearrange my furniture after getting another upgrade - and one of my villagers (I think Sprinkles) came in. I was just standing in the middle of an almost completely empty room thinking 'this is great and all but not really the best time'. she still complemented my house though so... that was good.
I've had it happen once with Kyle, it was really surprising! I had my doorway blocked with my two toilets though so all he could do was run around in the entrance for a little while.