For a long time (guess since 1.6 or 7) I have used certs to control students class to class movement distributing next class link and key but I have one school where it just seems like every couple of months the school would call and tell me the the links were gone or 'the students all say the course keys are not working' I'd check and everything seemed fine.
In the last few months their enrollment has tripled so that that occasional call grew to a couple of calls a week so with thier permission I called a few students to find out what they could be doing to mess up th course key that I had use custom text to the key under the words next course key below.
Turns out they were trying to use the verification code as a course key so I made a little blue key outline and put it around the key. That seemed to end the key issue but I would still get a call that there was no link to the next course. Again I spoke to a couple of students and found out they had no idea that the words click here was supposed to indicate a link to click on or maybe they just did not read it.
Anyway I did a 1 month test using a more graphic set of instuction on one class. It was lke magic how the calls just stoped but I really had no intention of doing it for all of thier classes. Better they should learn how this was supposed to go!
Anyway here is the video I made showing them how to duplicate my fix in all of thier classes.