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

int a[0100100];

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 :)