Oh, do not get me started!

'Two times' instead of 'twice' ("Our Brand kills two times as many germs as Brand X!").

'Stadiums' instead of 'stadia' ("Sports stadiums all over the country").

Confusing 'borrow' and 'lend' ("Can you borrow me a fiver?").

Peppering ones' discourse with 'so' and 'like' for no apparent reason:

"So I was like going to work and just as I was like getting on the bus I ran into Sarah. So Sarahs' like going out with my brother, but he like went to the match Saturday instead of taking her out. So she was like moaning to me all the way into town!"

You got me started! :)