If you need to guess the common SEO mistake which mostly made by the optimizers, Then what you will guess? Usage of images or Flash in making a website. Writing a title tag which is not verses home and the keyword phrase. Spamming Google Search engine? Do you agree or not.
If you are not agreeing then,
Matt Cutts, Google’s head of search spam, said it is not having a site at all.
Matt Cutts goes invisible in one of the funniest video at some portions and he says it is the mistake done which the first one. Google does not know whether you are exited or not when you does not have any website. Therefore, it seems Google cannot know about you. It is suggested to get a website.
Here is the Video: