A locum doc diagnosed heat rash for ds2 when he was 7 months old. It was eczema all over his body due to being allergic to wheat! He looked like a burnt lizard.
Having said that, it was a locum who finally diagnosed dd1 as having whooping cough, after seeing 5 docs before, and me saying to dh she had it. He was only able to say it was, because he had worked in countries where it was v common. Most docs here don't come into contact with it apparently.