I can't tell you the number of times that a student has forgotten his or her notebook at my studio after a lesson. It happens all the time. And, embarrassing as it may be, it's even more embarrassing to show up to your next lesson totally unprepared because you couldn't remember what to practice.
So what should you do if you forget your lessons notebook?
Call your teacher immediately. If it's me, just give me a buzz and leave a message, if you have to. Most likely, I'm in the middle of another lesson, a gig, or a recording session. In any case, call as soon as you find out you left it.
Practice what you can remember immediately following your lesson. If it's impossible for you to pick up your notebook right away, it's really important to practice as soon after your lesson as possible. Doing so helps to solidify all of the new content from your recent lesson.
Arrange a pick-up time ASAP. This one is pretty obvious.
Call if you have questions. Okay, yeah, you forgot your notebook. Yeah, it was a stupid thing to forget. But it would be even stupider to not practice because you couldn't remember what you worked on in your lesson. If I'm you're teacher, you can always call and ask questions.
All teachers do things differently, but I like to try to make myself available to my students via phone or email throughout the week, just in case they have questions about their homework. Give me a call. Let's try to work it out over the phone.
In some cases, I have also been known to hand-deliver forgotten notebooks to students. Again, not standard protocol, but I do it every once in a while, if I have the time and I have your address.