Adwords query

Advert test

Has anyone heard that running a Google Adwords campaign affects your SEO ranking in a negative way?

I've heard a couple of anecdotal stories from two small businesses (not from techies) that after a short Adwords campaign that their search ranking got worse. I don't have any figures from GA or GWT to back this up.

It sounds unlikely... Just wondering if anyone has heard anything along these lines or is it just daft?