This slip-up occurs at the part of the movie when the Rangers are fighting the Oozemen, after they have morphed.

When the fight scene gets to the part where we see Rocky (the Red Ranger) and Adam (The Black Ranger) being pursued by a couple of the Oozemen. At the end of this little segment, Rocky states "These guys don't know when to quit" as one of the Oozemen stand up. Rocky then proposes that they do a "Double Whammy", or in other words attack the Oozeman from both sides. While Adam is on the screen, showing that he is replying to Rocky's question/idea, the audience hears "You're on!" being spoken by Rocky instead.

So, basically with this slip-up, Rocky replied to his own question although the character that was shown, who was supposed to be talking, wasn't the Red Ranger.