(Moodle 2.4.5 ... probably later versions too)
We have noticed that the row number logic used in the assignment module is somewhat dangerous. At least if you are working in multiple browser tabs and is about to grade a student.
If you are filtering the assignment list differently between tabs and if you end up trying to grade someone in one of your "old" tabs where you did not filter the list most recently, then you might get into trouble. First of all when you click the grading link (.../mod/assign/view.php?id=100&rownum=14&action=grade) you could end up grading another student than the one you just chose. That is because of the row number logic used in the filtering. Since you can filter the list in other tabs row numbers will change in your session.
So if you are not observing that you are about to grade someone else than the one you intended then the wrong student might be graded
You might also happen to filter the list in a way so that you try to reach an index which is not currently present. But then you will actually get an error message like this one:
Coding error detected, it must be fixed by a programmer: Row is out of bounds for the current grading table: 1…
Quick grading however seems to be working fine, because of the userid logic instead of row numbers.
I just wanted to tell you about this, perhaps you would like this to be reported as a bug.