Of course I remembered Musaf--I've heard and prayed it a few thousand times. I also had to learn most of these same melodies when I lead Shacharit last summer as practice for the High Holy Days. I got to the bima and opened the siddur, and the rabbi leaned over and whispered, "I'm sorry to put you on the spot!" No problem at all, really. Any time. I love to hear my heart beating like a very loud dumbek.
It was fine. I had stood in that same spot, and wrangled that same microphone, five times over the past six months for Friday night services, and twice over the High Holy Days. It felt like coming home. But I was definitely tense, as can also happen when you encounter your family. We got to the end of the Aleinu, and the rabbi gestured for me to sing the last line. In my own siddur, which is covered with pencil scribbles reminding me to BREATHE and RELAX, that line is marked with a little red arrow. And I know it by memory. But at that moment, I could neither remember the the words nor find them on the page. Nerves can scramble one's perception, as happened to me many years ago during a performance at an ensemble singing workshop when I lost my place, looked down at the music, and realized I could no longer read music. I stood on stage and smiled for eight minutes, not making a sound, after which the vocal coach, well aware that we were amateurs on vacation who paid a lot of money to get positive feedback, said, "I've never seen anyone stand in one place and smile better than you just did!"
In this case, all I had to do was shake my head slightly and step back, and the rabbi sang the line. I felt completely naked, and very safe. When we got to Kiddush, after all the announcements, he handed me the large silver cup. "Do you know the words?" he asked, over the sound of the keyboard intro. I wasn't sure. I'd heard them every Shabbat morning for five years--but I'd never actually read them, and it's different from Kiddush done at home. (I think.) "I'll help you," he said. I held the cup high and read his lips as we both sang.