As the long May Challenge started and so has the cases of unfair means over some famous forums and what not! People have already started asking for help on various forums, for example:
Here (Link removed till contest is Live), a user is frustrated with the Matchsticks problem so wants to seek some help.
I have few more examples, that I would reveal after the contest.
And even after a lot of warnings for their own sake some people won't use private settings on online IDEs.
If you find any such case, even after the contest, use this thread to report each and every such case, so that admins can easily have a look post-contest and I hope this will act as a potential threat to those who are willing to cheat in future.
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Edit: Links to such cases to be posted after the contest. If other users are going through those links with a hope then can't really help.
UPD: As the contest is over, so the relevant link is added back to the question.
Edit2: Here is another example of a couple of users fiddling with a normal guy to elaborate the required solution. Check the comments.