> Do you mean "should've"? That's a common contraction of "should" and "have." In many American accents, the difference between "should've" and "should have" is negligible, and will sound like "should of" even though it isn't.

I think they specifically meant "should of" which is a colloquial form of "should've" in a number of places in the UK.

I went to school with a large number of people who would write "I should of done X instead of Y". In fact I'm pretty sure I made that "mistake" a number of times growing up.