SEO is a complete different world and it has its good and bad. Getting your self top rank in google is like a on-going challenge as we always have to keep up with the google search engine changes. If you do not keep your standard of optimization up to date with google changes, it will harm your ranking. Also I have summarixed 10 SEO bad practices that can harm your google ranking below:
1. Cloaking
2. Link-exchanges
3. Duplicate Content
4. Buying Links
5. Keyword Stuffing
6. Hidden text
7. Link farms
8. Writing for search engines
9. Over-optimization
10. Low-quality content
Keeping these things in mind might help you stay out of trouble harming your google rank.