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A few people have posted similar cases lately in the forum. Usually it is a result of spyware being on their computer. Google does not "do" popups.....so there are a few reasons as to why you seen one
1) The site you were surfing previously uses an "exit" popup for whenver you leave their site
2) Spyware that spawns a popup whenever you go to your home URL
3) A.N.Other spyware that shouldnt be there.
i.e. check your browser for spyware or the likes. It may have been a one off........but in the end of it all, Google claims to not do popups so I guess the best avenue to go down is to check your browser just in case.
Some people have mentioned a program called "Adaware" that scans and removes spyware from your harddrive (in the event you have it on your comp)
Hope that helps