Posting a code snippet instead of a complete answer
Note: There is now a separate question covering this loophole in more detail.
This is a issue mainly for code-golf, code-challenge and variants.
Only in very few cases this seriously affects popularity-contest.
The issue is when someone post a function, a method, an instruction or a code-snippet instead of a complete program. And then score his/hertheir answer based on only what was posted. This gives an unfair advantage over who posted a complete solution.
Here is an example of a complete C Hello World program in 53 bytes:
#include<stdio.h>
int main(){printf("Hello World!");}
But, the answerer, with the intent to defeat another answer in code-golf, posts this instead, claiming that his/hertheir answer is 22 bytes:
printf("Hello World!")