From WaPo:

McCain said it was “common knowledge and has been reported in the media that al-Qaeda is going back into Iran and receiving training and are coming back into Iraq from Iran, that’s well known. And it’s unfortunate.” A few moments later, Sen. Joseph Lieberman, standing just behind McCain, stepped forward and whispered in the presidential candidate’s ear. McCain then said: “I’m sorry, the Iranians are training extremists, not al-Qaeda.”

The mistake threatened to undermine McCain’s argument that his decades of foreign policy experience make him the natural choice to lead a country at war with terrorists. In recent days, McCain has repeatedly said his intimate knowledge of foreign policy make him the best equipped to answer a phone ringing in the White House late at night.

Update: Heeeeere’s the video:

Bonus: Apparently McCain knows he’s supposed to say bad things about Iran and Al Qaeda, but he’s just conflated the two. He made the same mistake yesterday on the radio (h/t HuffPo):

As you know, there are Al Qaeda operatives that are taken back into Iran, given training as leaders, and they’re moving back into Iraq.