Possible "Speed Bumps" for SE Spiders(Search Engine Optimization):

URLs with 2+ dynamic parameters; i.e.

http://www.url.com/page.php?id=4&CK=34rr&User=%Tom% (spiders may be

reluctant to crawl complex URLs like this because they often result in

errors with non-human visitors)
 
Pages with more than 100 unique links to other pages on the site

(spiders may not follow each one)
Pages buried more than 3 clicks/links from the home page of a website

(unless there are many other external links pointing to the site,

spiders will often ignore deep pages)

Pages requiring a "Session ID" or Cookie to enable navigation (spiders

may not be able to retain these elements as a browser user can)
Pages that are split into "frames" can hinder crawling and cause

confusion about which pages to rank in the results