I have two but neither are from BA.

I'm sure this is an oldie but I've only heard it once. A CO FA just after the door was closed announced "Welcome to flight xxx to Newark, if Newark was not your destination today... well, it is now!". Maybe you had to be there.

But the really funny one was just unbelievable. In French with an English accent the captain announced that they had new Monty's snacks. These were available in Vegetable or Lamb and Mint flavours. Except he didn't get one of the words right. I can only suspect his coworkers were playing a practical joke on him at his expense to poke fun at his lack of French language skills. He announced that the second option as "Agneau et de la merde". A few minutes later he came on the PA again to make a small correction...