I have no idea what I am doing
This is a strategy that works very well near the end of a topic, tell the students at the start of the lesson you are going to be wrong five times in the lesson. That you, as the teacher is going to make five mistakes and the students have to point them out. You can do this in two ways.
During the lesson – give the students 2 chances to get it wrong. Tell them that when they hear a mistake, they have to put both hands up, this can lead to some very good class discussions about why some students believe some misconceptions. However, they only have one chance to get it wrong.
Once they have made a mistake they are out. At the end of the lesson you show the students which mistake was made and why it is a mistake, you can see how many students make it to the end.
At the end of the lesson – if you do not want your lesson to be interrupted for this strategy ask the students to jot down the mistakes and the time of the mistake. The time makes it easier to “mark” at the end of the lesson. This works well as a plenary of the students reacting to parts of the mistakes.
With both of these make sure the student explains why these are mistakes, what about the answer, working out, the diagram was wrong, this will help solidify the understanding in the room. This is a very simple way to add challenge to the lesson. You can make these mistakes as obvious or as tricky as you would like.