First up is including html in a page's title tag. Wanting the title to be bold or italicized or in ALL CAPS is one thing--including other tags
accidentally is another. Altoft adds that you should also make sure that your redirects (temporary or permanent) don't ultimately send users to a 404 not-found page or some other, unfathomable error.
Don't force the engine's spiders to use cookies to crawl a site (even inadvertently), as they won't understand the rejection and will continue on their way, not bothering to try to index your site. And Altoft says to avoid putting date info in the h1 tags, as other site owners and bloggers will be less likely to link to seemingly outdated content--even if it is relevant and valuable.