Well I always thought that "should of" was a spelling error but I was at the local shop when the self-service till refused to let me complete my order (probably because of the couple of £1.30 Spitfires I'd got and I clearly look underage). Anyway the manned (ladied?) checkout was clear so I went there rather than wait for assistance and said "The machine's playing up, I knew I should've come here first" and she said "Yes, you should of". I despair.