In fact, this is happening to me right now around the third time in the row, and it still is interesting. However, in our case, this is happening, because this

In fact, this is happening to me right now around the third time in the row, and it still is interesting. However, in our case, this is happening, because this is the third time in a row we organized and talked a professor to give us a lecture around 6 people (which, at the moment, is 1/3 of all students in this term and study course). The conclusion is that the professor is not that prepared, and because the entire course is pretty math-heavy, errors in proofs and lemmas just slip in. Pretty often, they are just little typos, but it still is fun to hunt and spot them.