Another couple of slip-ups I have found, in Season2, episode 2, when Brian is talking to Kenny beside the inflatable, his phone goes off. He has a Motorola phone but it was a Nokia message alert sound.
Secondly, in the first series, when Brian is yelling at Alan in the first episode, he calls him Alan 'Stranraer' Johnson (his middle name is an insult to the fact that he was late to the club) but later on when Les Alanos are performing 'The Karate Kid' show a poster outside says Alan's last name is Dalton.