If a page was previously ranking well for significant terms, what would be some reasons it had become un-indexed?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Chrissi Reimer: If a page was previously ranking well for significant terms, what would be some reasons it had become un-indexed?
  - Nothing in GWT
  - ranking fine one day then dropped off of Google the next day
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  Unique Websites: I would guess either unnatural links (especially those with keyword anchor text) ;OR spammy onsite SEO.
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  Greg Kristan: Sometime rankings just will drop and then jump back to their same position. Or it could what ;+Unique Websites said.
  
  If you are really concerned about that keyword then you should run a link detox report to see if you have a lot of external links with that keyword in the anchor text. ;
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  Unique Websites: +Greg Kristan ;Is Google more likely to penalise for exact match keyword links - or is just having one keyword (eg location) in a reasonable amount of anchor text links likely to be a problem?? ;
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  Greg Kristan: +Unique Websites exact matched keyword links are a large reason for penalties. This raises a flag that these links are unnatural 
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  Chrissi Reimer: Thanks for the thoughts. The major issue is that the page is no longer indexed. There are very few external links pointing to the page. We're looking into internal links and their anchor text. Still, it does not appear to be too dramatic of a number of links. I'll let you know what we find. :)
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  bharat thakare: If there's no message in GWT then there could be many other reasons. I would look at google analytics to find out on which date there was a drop in organic visits esp. Google. I would then find out if there was any major change on the page which could have led to the page getting de-indexed. Did you check your robots.txt file to see if the page is blocked? Also check if the page has robots meta tag added mistakenly. I would also recommend checking backlinks of the page.
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  Unique Websites: +Chrissi Reimer ;One other thought - does the page have unique content? If it is just a rewrite of another page, then - possibly?? - that might cause deindexing?
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  Greg Kristan: Good point +Unique Websites