Nothing to worry about. Just hand this over to the immigration officer when you get back here. Same thing happened to me a month ago. They forgot to take it off and when returning I let the officer know about it and he kept the my old i-94 and gave me the new one. To be on safe side, they suggested me to have the boarding pass of the flight in which I left the US (But nobody checked that one too. So, if you dont have it dont worry about it too.)

Jhilke :)