A note about cache. The way to make cache links moot in Google is with the noarchive directive.
<meta name="robots" content="noarchive" />
Fact: according to a 2007 internal Google document given to quality raters Google will rate your site based on the google cache copy if for whatever reason your page doesn't load. Since Google's cache may very well be outdated when they go looking it's probably best to make sure there is no Google cache.
Snippets from the document - instructions for google's quality raters. (emphasis mine)
#1 - It's nice to know someone is worrying about your ratings and hard work.
Please go with your best judgment and do not worry too much about rationalizing every single rating decision
#2 - It's nice to know you'll get the benefit of the doubt for that hard work.
Sometimes you may feel unsure which of two ratings to give: Relevant or Useful? Not Relevant or Relevant? When you are unsure, select the lower rating.
#3 - About cache and how it's used to rate your content, or sometimes content on Google instead.
IMPORTANT: When to rate live vs. cache: If live page loads . Rate the live page . If live doesnít load, but cache loads . Rate the cache page . If neither cache nor live page loads . Rating is Didnít Load .
So, if we follow the same logic, when we're not sure about Google cache we should ensure it's not used. Legal Disclaimer: I do not have a copy of this pdf document, it is freely available online and I found it posted in a public place. I have copied snippets under fair use for the purpose of discussion in a public forum only and do not claim copyright(though I do hope it's seriously outdated by now).
Moderator's note. Link to 2007 Google Quality Rater Guidelines in this thread.... [webmasterworld.com...]
P.S. the way to stop seeing seemingly random tests is to stop using Google. A recent google engineer video states that 2.5% of results are in fact a test.
[edited by: Robert_Charlton at 10:40 pm (utc) on Sep 24, 2011] [edit reason] added link [/edit]