In business not everyone gets along, generally in SEO were a friendly bunch but I’ve been in the middle of arguments between clients and ex-web designers. The disagreements are usually about payments, missed deadlines or lost contracts.
I’ve seen on three occasions web developers get their own back by making little changes to stop sites ranking in Google.
So what can you learn, give all third parties a temporary FTP access to the site, when they finish work delete the account. Check after work your htaccess file and for any Google Webmaster Tools verifications (either a html file or a meta tag in the home page).
The only way to avoid any problem is not to be a dick and annoy web masters or SEO’s!