There is a problem I notice recently When hacking, for example, some people will hack others to see others' code after they have done such as D question. At this time, some of his friends may come to him to ask for the solution of D question, but many people do not want to take the risk of their own solution being judged cheating. They might send someone else's code which copied from hacking other people to their friends, and if their friend are judged cheating by system, the one who be hacked is really hard to prove he is innocent. So I think this problem is really serious.

And I also have some suggestion Firstly, the codeforces might can record the data about the user who view your code during the contest, and if there is any problem, it can provide some evidence

Second solution might be really useful but It also would cause a lot of problems , moveing the whole function of hack after the contest end, even someone send code to their friends, it is not count in their score, It might be a way to prevent cheating. But this method still have many aspects to improve, It is really welcome for everyone to give some own opinions.