Plenty:
- duplicate content
- keyword stuffing
- black hat link building strategies (spamming with back links)
- spammy crappy content (built for adsense)
...to name a few.
However, if you've recently built a site and got indexed, it is normal to disappear from SERP's in the following month or so. You'll be back soon after google calculates your SERP (Search Engine Results Page) position better.
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