Lack of research!
I was watching the episode where Pinky arranges a surprise birthday party for Brain and invites all the wrold's leaders icluding Bill Clinton (who wolfs down a pizza) and John Major (then then Prime Minister of Britain).
They get John Major's voice COMPLETELY wrong! In reality he spoke very nasally and 'little man' like, kind of like an English Hardy Har Har or one of these wimpy Don Messick-like voices, but in this episode of P&B they have him speaking in the predictable 'I say, don't you know old boy!' accent.
It kind of stuck out like a sore thumb in an otherwise very intelligent show.