fluff (one's) lines
To say the wrong words or forget the words entirely while reciting something or performing in a play. She had been doing so well in her debut performance before she completely fluffed her lines in the final scene. I had learned the speech off by heart, but I was so emotional that I fluffed my lines halfway through.
fluff one's linesand blow one's lines; muff one's lines
to speak one's speech badly or forget one's lines when one is in a play. The actress fluffed her lines badly in the last act. I was in a play once, and I muffed my lines over and over. It's okay to blow your lines in rehearsal.