I've noticed something weird with the cart in 2.7. If someone backs out of a form it still adds it to their cart. I believe this may be more of an issue when people are using IE. I went to correct my captcha at the bottom of the screen and hit the backspace key one time too many, causing the screen to go back. Is there any way that events can be kept from adding to the cart until the user goes all the way through?
The new package of 2.7 includes various fixes for the cart. Please update and see if this issue remains for you. If so, submit in a ticket if you haven't already.