The clearest case of this I've seen was Norvig's Udacity CS212 class: in the third week there were despairing and even angry posts to the forum about his code to generate strings from regular expressions. (Here's a version of it, with small changes plus a regex parser, since Udacity started requiring login: https://github.com/darius/parson/blob/master/eg_regex.py)

But some people have also told me that was a great learning experience -- they spent many hours understanding this half page of code, and felt they'd really grown once they'd mastered it.