Why is Google ignoring my Title tag and meta description?

Since late 2011 we have noticed ever increasing examples around the internet of Google ignoring site title and/or meta tags and instead displaying their own ones. This is happening to many sites around the internet and seems to becoming more and more common.

Some of our clients and other website owners on discussion forums have reported one or the following things happening to their Google listings:

Title tags showing that are not the title tags that are set for a page and what appears to be 'automatically generated' title tags by Google appearing in Google search results
Meta descriptions not showing and instead Google is showing other content from within a page. This seems to be happening a lot with Ecommerce sites showing lists of categories or products rather than the actual meta descriptions

At Mantis Technologies, we have done extensive testing and research into these issues and have also found another interesting occurrence. Some pages report different title and/or meta tags based on what you are searching on.

It seems that Google is deciding what is more 'relevant' to the person searching and will often show different title and/or meta data in their search results that more closely relates to the search results.

Although it is not known exactly when and why Google does this, we have found there are some things you can do to lower the chances of Google changing your title or descriptions in their search results:

Keep your title tags short. Some experts suggest under 45 characters. We suggest around 3 to 5 words is good. Also remember the more words you have in a title tag, the more each word's effectiveness is diluted.
Keep your meta descriptions short. We recommend stick to one sentence descriptions.
Ensure the heading (h1) of a page is close to or even exactly the same as your title tag to reinforce the title tag.

Our SEO research and testing will continue but we have found in all our test cases, pages that follow the above 3 rules don't have their titles or descriptions changed in Google search results.