Ok, so why not give the user a warning rather than leaving us to wonder what’s going on? This is right up there with the “Failed to save Lesson” message. The user needs to be informed of the error and given a reason why so he can self diagnose. Just throwing the lesson in the trash makes no sense.
Besides: the url is only one small part of the lesson. In this case, the audio and text for both lessons are located at the same url yet the audio and text of the two lessons are completely different.