Canon 7D problem?
 

Canon Rumours are reporting a possible fault with the 7D where it is used in high speed mode the previous image may be imprinted on the following one. Canon are aware of this and looking to solve it via a firmware update.

An official announcement is here : [url]http://www.canon.com.au/support/customer/default.asp?DXI=KnowledgeBase/Customer/KBArticleForm&File=KB01685&productID=eos7d_body[/url]

I`ve not come across the problem as of yet, but you never know!