Some people, desperate to rank well in Google will go to any lengths to achieve page one status. 

One such error in judgement is the hidden image text that is exploited on many sites.  For instance, see the above image?  Hover over it and you’ll see I have tagged it with an appropriate keyword that will help me with SEO, but more importantly will let readers who cannot view it know what they would be seeing if they could see the image.

Here’s what Google’s Matt Cutts has to say about it:

You may just acheive positive SERP results by playing the ‘black hat SEO’ game, but you will evenutally be banned and not index at all in the search engines.