The teacher was prepared. She had put the agenda up on the board before she left for the weekend. That's where I noticed the error. The lesson plan said that the class was to outline chapter 10.3, but the board said that they were on chapter 12. Oops.
Unfortunately, this was the 9th grade class.
9th grade is a weird time. They're in high school, but I've found that the more I treat them like middle schoolers, the easier my day goes. And they think they know everything.
When they arrived, I explained the situation. I told them that I knew that they weren't in chapter 10. They told me that I must be reading the lesson plan for the wrong class.
Yes, the 11th graders were in chapter 10. That was the 5th period class. I could tell the difference between 1st period (at the top of the page) and 5th period (at the bottom of the page). I wasn't reading the lesson plan wrong. The lesson plan had the error.
(Usually, at this point I'd let them read the lesson plan, but I wasn't in the mood to take the page around and show it to each of the 39 students in class.)
So, I told them to outline chapter 12.3. But now this was wrong. They were only in chapter 12.1 or 12.2 (it changed depending upon which student was speaking). Their teacher had started giving them notes, but she hadn't finished. My suggestion that maybe she wanted them to do the next section on their own and she'd finish giving them notes on the other two sections met with harsh disapproval. No, she didn't mean that.
I told them to outline something. Anything. If they weren't going to take my interpretation of the lesson... (Which they should do, really. If I tell them the wrong thing to do and they do it, I'm the one that gets blamed for the error.)
At least I only had one class of 9th graders. And the rest of the lesson plan contained no errors.