On some cases where exam results are manipulated This problem has been bothering me as a lecturer and future PhD student for a while now.

A student just finishes filling his exam sheet. He then gives the sheet to the lecturer. The lecturer is then grading the result. He then returns the result to the student.


(Bad student) After seeing his score, the student does not accept it, but he knows this result is fair. He then reports to the person above the lecturer that his exam sheet has been changed by the lecturer (which is possible, isn't it?). He claims that this is not his exam sheet. He then argues and keeps arguing by bringing made-up facts and made-up evidences.
(Bad lecturer) This case is similar to case 1, but this time the lecturer has indeed change the exam sheet. The student is then surprised and he can't prove it wrong somehow.


How can we prevent these two possible cases? I have seen case 1 but that was a long time ago and I forget how it was solved. However,  I think it is fine to prevent bad things before they happen rather than handling them after they happen which usually turns out to be too late.

One solution I can think of is going online. Do test online, share exam sheets online, etc. Is there another countermeasure?
There is no counterneasure which is bulletproof and administratively feasible. If I want to cheat as a teacher, I will always find ways around such a system. 

The only way would be to have the exam taken by independent administrative staff, they scan all answers as proof, and then hand it to the teacher. 
Online results could be watermarked (e.g. by an MD5 / SHA hash), but I don't know if such systems exist. 

If I really want to upgrade or downgrade a student, I would not take the risk of manipulating the exam, but to adjust my measures for deciding what is true and what is false and to which account I'm willing to accept "a good idea which did not lead to the correct result". 

As a teacher, I like to have electronic submissions since they are a proof and students usually have nearly no means of changing them afterwards. But I prefer  doing the grading with paper and pencil.