When someone absentmindedly sings a crappy song over and over to themselves. If they don't know the whole song, they repeat the parts they do know, much to the annoyance of people around them.
Clerk: (quietly singing) "Ooh baby baby, how was I supposed to know/ something wasn't riiiiight..."
Receptionist: "Can you PLEASE stop that idiot singing! You have been singing Britney Spears for like 5 hours."
Clerk: "My God, you're right! I don't even like that song. Sorry."
...5 minutes later...
"Ooh baby baby..."