The following program will print a different value for a depending on whether it is compiled in C (pre C99 standard) or C++

#include <stdio.h>
void main()
{
   int a;
   a=4//**/2;
   ;
   printf("%d\n",a);
}

The reason being that traditional C reads the line as a=4/2; with a comment in the middle. C++ (and C99) comments out the whole line after the 4, and uses the following semicolon to end the line.