I've given you a little cheat for this challenge. You can watch the sketch without the subtitles and see if you can identify the mistakes, or you can download the transcript off the YouTube page and see if you can locate the mistakes that way. Either way or both are great exercises to sharpen your listening skills.
There is a British version of this same sketch (this is the American version) that you can watch here The Sketch Show UK. I personally didn't think it was as funny as the US version, despite being a huge fan of British humor. They are so cheeky, aren't they?
Native listeners will hear the mistakes instantly, like fingernails running down a chalkboard. But non-natives might have some difficulty. Give it a shot and let me know if you need any help!
Good luck, aren't they?