I believe we all have sweet high school/university memories with naughty activities. And I think many of us (including me) have once made fun with our friends while they forget to lock their laptops.
Here are couple of things I have added to my friends' code when they were away:
#define cout cerr
many may ask why they passed the samples but couldn't pass any tests on the server
#define printf(...) fprintf(stderr, __VA_ARGS__)
same as above, but for anti-fan of cin/cout
#define if if(true); else if
your friends will have great time debugging their code with some weird behaviours.
#define freopen(...) void()
extremely helpful when your judging system use input/output via files
just add a small zero, but the whole world can change.
What else do you do? Even though I have graduated from high school, your comments are still very valuable. And I honestly hope your friendships stay stronger after these things :)