Sometimes you will find that a question in a quiz is incorrect. This could mean that it has no correct answer, or it maybe was just too confusing to students to be able to understand clearly.
In these situations you may want to "fix" the issue but without penalising the students who may have got it right.
You can override the automatic marking of the question via the Results > Manual Grading facility.
Set the form to include automatically graded questions and you can page through all of the student answers for the question and give whatever mark you wish (and feedback too if you want).
However this means reviewing every student's answer to the affected question, or if you're using a random question from a question bank, every question answered by each student in the affected slot.
Instead of awarding full marks for the question, you can get Moodle to discount the question by setting the available number of marks for that question / slot to 0.
Moodle will automatically accomodate this by scaling up the other marks.