My experience is that they get about 45 minutes into the lecture, realize that there was an error in minute 10 and spend the rest of the period trying to fix it before telling you to check the book for the correct proof.

One professor provided scans of his written notes. For one of the theorems in his notes, he’d stated it, attempted to prove it, failed, crossed out the proof and wrote ‘QED’. At least he was honest enough to include it!