I guess he means they can't name ONLY one issue he has flip-flopped on

McCain: You ‘can’t name a single issue’ I’ve flip-flopped on since 2000.

During an interview this afternoon with Sen. John McCain (R-AZ), a reporter from local Washington, D.C. area CBS affiliate told McCain that some “commentators” and even some “personal friends” have asked, “Where is the John McCain from 2000?” The reporter then asked, “Did that guy go away? Has something changed?” But McCain took issue with the premise of the reporter’s question, claiming that nothing has changed since 2000:

MCCAIN: You’ll have to tell me what’s changed. I love it when they say, “Oh McCain has changed.” And I say, “What have I changed on?” They can’t name a single issue or they’ll name an issue and its false. I’m the same guy. I’m proud of our campaign.

In fact, ThinkProgress has identified at least 44 issues that McCain has flip-flopped on, including President Bush’s tax cuts, comprehensive immigration reform and torture.

(Think Progress)
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Of course, there's not just a single issue but there are several dozen issues, so maybe he's right that we can't name only one. There are just too many!