There are certain grammatical errors which are so enjoyable that they can become in-jokes between friends. This can be dangerous when outsiders are present, as they may believe you are making genuine mistakes.
Inspired by a trip to a local restaurant, my good lady wife and I now enjoy getting ‘an’ and ‘a’ the wrong way around. This, taken from their menu, is why:
Leaving aside the tautological content of this menu filler (why didn’t they just leave a space there? Which words could possibly add to the enjoyment of a meal apart from ‘Your meal tonight will be served by a wheelchair-bound Margaret Thatcher, who has just 15 minutes left to live. Feel free to kick her up the arse as you see fit and empty the contents of your pickle tray onto her rotting corpse’), just how did they get this so wrong?
They certainly aren’t alone. This appeared in the Guardian’s G2 section a couple of weeks back:
Oh dear.

Comment by Gez — 26th October, 2007 @ 1:45 pm
Comment by Dan — 26th October, 2007 @ 1:59 pm
Comment by Andrew Green — 26th October, 2007 @ 2:41 pm
Comment by Dan — 26th October, 2007 @ 2:52 pm
Comment by Gez — 26th October, 2007 @ 7:04 pm
Comment by Gez — 29th October, 2007 @ 1:26 am