September 2014
The Sim I'm interacting with was walking up the screen on the right sidewalk and was about to disapear at the end of the sidewalk... before that AI sim reached the end of the sidewalk I interacted with that SIM and click "Ask to hangout", so my SIM started running to her about 20 strides away, then that SIM disapeared when it reached the end of the sidewalk and I thought I lost my chance to ask her to hangout but to my surprise, her invisible self responded when I reached that location as you can see in the screenshot. Whops!
September 2014
Yes saving the game going back to the main menu and loading it again fixes it and she reappears, but the main issue is what causes this in the first place.
Thanks mileyvery-happy: