The answer to the problem in question was a list with 3 elements in it, where the grader allows extra credit. The student had one answer correct and received the following information:
* 33% of the problem was correct. (The status was changed to 0.33333.)
* A green bar, along with the statement: "The answer is correct."
The student saw the green bar and didn't bother redoing the problem. (They get two tries for the test.) Later, he found out that his answer was wrong.
I consider this a serious bug, and I don't recall WeBWorK having worked this way. I'm not doing anything fancy in this problem. (I've summarized the code below; I removed the code not related to answer-checking.)
$showPartialCorrectAnswers = 1;
[code to generate a problem instance]
$ans = List (0, $l1, $l2);
Context() -> texStrings;
ANS ($ans -> cmp);