1. Too many website visitors
Highly ranked websites receive significantly more visitors than those which are invisible to the search engines. In our experience, increases in visitor numbers from 200% to several orders of magnitude are normal.
Large websites with lots of images / Flash / downloads may find their hosting provision needs increasing. This is normally only a problem with websites on very old host servers. Cost implications are minimal.
It is likely that the amount of Spam received will also increase. This may be more of a nuisance.
2. Too many website leads / sales.
3. Extra costs.
4. Risks- There are risks associated with short term SEO or poor SEO work (or over-optimisation of your website causing it to be blacklisted and fall in search engine positions.