Living in North London our children grew up with the dialect "should of" instead of "should have" (done something, for instance) syndrome.
It was bad enough when it was spoken but I have also seen it written quite frequently.
The worst example I came across was in a Nivea advertisement on the Tube! I can't remember the exact details - it wasn't "should of" but something along the same lines, such as "could of",
One wonders whom they employ as copy-writers, editors and printers that it got through to actually being put up n the Tube carriages!