O.K. maybe you're right but, if put it on perspective, the title means that it's not about two summers ago now, but about the last summer and the vengance he wants because he couldn't kill them and they try to kill him again. At least this an excuse.
Nickole, actually, if you put it into perspective, if the title is referring to the stuff that happened one year ago, thus making it correct to be "last summer," it's still incorrect... because it says "I STILL now what You Did Last Summer." So, it is a slip-up after all!!!