Although I am exaggerating a bit, there are mistakes that can get your website banned, or penalties.
Most common things webmaster do may be all well for your visitors, with good intentions but remember spiders are computers. They are not perfect though.
Some of these things that spiders are programmed to detect and put a penalty to the website include the following list.
Keyword stuffing in meta tags, on content, in anchor text of internal links or backlinks, the use of redirects, hidden text, a hacked website, duplicate content, link exchange with banned site, use of subdomains with same content as homepage or a website with viruses. There may be more...
Remember these will cause a penalty, these are not things you do that won't help your rankings - but they will actually decrease your rankings.
There is a difference between a penalty, or something you are not doing quite good that may results in a difficulty in getting indexed for example.
To fix a penalty which most often is decrease in rankings, just find out what's wrong and stop doing it or fix it. A penalty on your site will then after some time be removed.
If on the other hand you cannot get your site indexed, that's not a penalty, that's something you are not doing right. Maybe it's brand new and it has only couple backlinks. Just learn more seo, to find out why and how to fix it.
In extreme cases a penalty can result in totally removal of your website from the index. In the case like for example your site is hacked, or has viruses or adware. You have to fix once again your site, Google even warns you of such events if you have a Google Webmasters Tools account, and makes sure that once you fix your site is back in the index very quickly.
If you don't fix, if you don't know what to fix then you remain out of the index, of course. It's bad for the searcher to find a website that's hacked. That's why obviously your site is removed.
As an seo you need to remain up to date on what's going on in the seo world. Grab the wisdom and knowledge, so you know what's right to do and what's wrong to avoid.
Search engine algorithms may seem to be changing often, but truth be told nothing really changes. Only slight changes, like for example, if before keyword density was important, now it's more good idea to have good quality content.
So content is still very important, what can change may be that search engines will prefer a site completely with alt tags, rather than one without. And things like that. Your goal is to give search engines what they want.
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