What are the most common false positives when people think they're penalised?
http://dejanseo.com.au/false-alarms-google-penalty/

1. Blocking search engines in robots.txt / meta
2. Website redesign, changed URL structure, no 301
3. Server down or unreliable and slow
4. Authentication required to see the site
5. Running two sites with same content
6. Slight drop due to normal fluctuations or loss of a few links
7. Change in Google's algorithm
8. Poorly executed canonicalisation or URL parameters
9. Set the wrong country in Google Webmaster Tools
10. ?