Playing limit not working

This is a strange one.
I have a quiz with a playing limit of once per day. But recently a user was able to take the quiz about 15 times in one day. This shows in Results in the backend and also in a log
that I generate in a plug triggered by the onQuizCompleted and onQuizFailed events.
I tested it myself, and it did the right thing, telling me to wait until tomorrow after I failed.
How can I gather information to help you figure this out?

Hello John!

Well, was he logged in? Could you share a screenshot of his results, without disclosing personal data?

Thanks for posting.

Here is the screenshot for Results.  

Hi again John.
If the user is not logged in, we basically rely on cookies for setting a playing limit. So, if the user clears his cookies, then he can take the quiz again.
If this specific user was not logged in, (I cannot tell from the screenshot) then we should investigate further.

Any suggestions are welcome.
Thanks again.

User was logged in. The quiz is set to require login. Also, both your results in the back-end and my log intercepting the events you generate have a userid.
Any other info I can gather? I've been trying to think of something useful without success.

I am seeing this with other users, although infrequently. I contacted one of them and asked how he did it, but I got no response.
I have tried a few things myself, including logging out, clearing browser cache, logging back in, but it all has worked like it's supposed to.
I can send you offline a dump of the results table for one of the users, if that is useful.