I could of said "should of", I suppose.

Of course you could of, but could of's cool; we all know what you meant after all, which is the main thing.

And the tennis ball comes "off the racket" NOT "off of the racket."

It does in London, WAV; I was once soundly ordered off of the bus by a justifiably belligerent inspector for not having a valid ticket (hey, it's always worth a try) for my journey from Camden to Clapham on the 88. Made it far as Stockwell anyway...

Haddaway & Shite

I'm onto them just now to see if I can make a valid case for cultural discrimination.