The Google Metadata Web Authoring Statistics might be a bit outdated in a sense that Google pays more attention today to meta description element than it did when the article was written and totally disregards meta keywords element but it is still worth reading about the common mistakes.

The http-equiv values pragma and expires are attempts at bypassing caches without having to set the HTTP headers correctly. These are probably unnecessary uses; any scenario where there is a legitimate reason to limit caching, the author is going to have enough control over the server to send the appropriate headers. In addition, the meta tags can't be considered reliable (e.g. proxies and transparent caches aren't going to honour them).

The distribution value is supposedly used to control who can access the document. Search engine "optimisers" tell people to set it to "global" to ensure that search engines index their pages.