It would be nice if there was a way to flag a particular question as extra credit, so that's it's total points would not affect the overall points possible, and if the student got it right, they'd get the points, but if they got it wrong it would not affect their grade.
Also, if the student got 100% on all the other questions, there'd have to be a check to make sure that the extra credit is not applied because it's not necessary.
(Although it would be interesting to hear use cases here. If the test is worth 20 points, with 2 extra credit points, and the student gets all the questions right, should they just get 20 points added to their overall course grade, or should they get 22, even though the test is only worth 20? Does the extra credit only apply to the test, or does it apply to the course grade as a whole?)