I finished a CoC earlier today in which 4 points where given and the task was to decide if the points form a rectangle. If A, B, C, D are the points, I just checked if (A-B) == (C-D) and (B-C) == (D-A). This solution gave me 100% even though it is wrong. I just checked the opposing sides so that a parallelogram is also accepted.
But there is obiviously no test case that is checking this case. Something like (0,0), (1,10), (11,10), (10,0) could be added to the cases.