A big mistake to avoid is repeating keywords in your head tag more than one time. It is a complete myth that the repetition is beneficial to you. There is no benefit to the repetition regardless of what some self appointed gurus may espouse. Google and Bing have even said this on their official blogs and webmaster tools guidelines. This is not some secret being divulged here. It only takes using a keyword one time to clue in search engines that your site is relevant to that keyword. Keep it simple and clean and avoid repeating keywords in your head tag.
Another biggie to avoid that is way overdone is using Flash. When you build your site using Flash, you are putting one more thing out there that search engines cannot crawl. Granted, Google is working on crawling Flash at this time and is having some success with it, but it is not doing it well enough you want to gamble you’ll be one of the sites it not only works on, but works correctly on. There have been issues with Flash sites being crawled but mis-indexed. Staying on that theme, if you are using frames, the odds are you are once again losing out on some search engines crawling and indexing your site. They are not necessary so you are best advised to avoid using them.
When it comes to Meta tags, limit it to the keywords alone. Search engines have no interest in Meta Tags like author, date, or things of that nature. Keep it simple and just stick to the keywords. If you flood out piles of Meta tags, eventually all your tags will be ignored because your site will be labeled as a spammer and it won’t matter what they are anyway. Mass Meta tagging once worked, but those days are long gone and Googlebot is much smarter now than it was then.
On a final note, even legitimate redirects are going to be viewed as a malpractice. When a page redirect takes a visitor to a page other than the one he/she intended to visit, search engines will not be able differentiate this. As much as any SEO mistake, this is one you absolutely have to avoid. Re-directs are fine when used for what they are supposed to be – a temporary patch. Using them as a permanent fix is not a fix and it is not going to help you in the rankings or with your PageRank!