What is wrong with that?
Well, setting aside the obvious insulting nature of the accusation mischief, it was made in the context of me lying about how long I'd been in the UK.
Immigration Officer: You have a British accent.
Me: Do I? I've only been here about 5 months.
IO: No, you must've been here longer to develop that accent. *You're lying*
Me: Well...I haven't...
This isn't the most pleasant situation you could find yourself in.