When a group of friends is talking and a laugh occurs, while still smiling you make direct eye contact with a stranger and it quickly becomes creepy.
You're friend Brady makes a joke while you are eating in the cafeteria.
"What do a pimp and a pirate have in common?"
"They both say Yo-Ho, and walk with a limp"
while still laughing you look away and make direct eye contact with a red head across the cafeteria. (this person is receiving your residual smile)
Later, when you see that person again you feel obliged to say:
"I'm sorry that I smiled at you"
and the whole thing turns creepy.