The error message when a quiz attempt can not be created is HTML escaped when it shouldn't (at least the <p> tag, as you can see in the image). I found it by chance because I opened twice a link to solve a quiz (and as it only had one attempt it displayed that error, which is normal).

I have attached a screenshot of what I found (the language is Catalan). The "more info" link points to http://docs.moodle.org/401/ca/error/quiz/attempterror , in case it could help debugging.

STEPS TO REPRODUCE:

1. Create a quiz with a single possible attempt (Grade → Attempts allowed)
2. As a student, open the quiz page (where you can see all the attempts) in two different browser tabs (tab1 and tab2).
3. Solve the quiz in tab 1 and send it.
4. Once finished, start the quiz in tab 2. The error page from the screenshot will appear. This behavior is expected, but the error is HTML escaped (the <p> tags can be seen).

I'm not familiar enough with moodle to say if this happens only in the catalan language or not, or if this has been already fixed. It happened with version 4.1.9, which AFAIK is already a little bit behind the current version. Feel free to close it if it has already been fixed