I tell my students to check over their tests very carefully after I grade them and hand them back. I am only human, and I can accidentally mark some of their answers wrong when in fact they are right. Once I am made aware of my mistake, I correct it immediately and add to their score. Sometimes I mess up the other way, though. Thankfully it doesn't happen much, but I have marked answers right before when they were actually wrong! For the student, that is just the luck of the draw. I will always grade in their favor. I will add to their score when necessary but never will I subtract from it.
Jesus promised us that if we believe in Him, He will give us Eternal life. John 3:16 is just one example of this Truth. But is it possible to sin our way out of that gift? Is it possible to lose our salvation? I believe with all my heart that the answer is no. Can our perfect God make a mistake? Would He give us Eternal life one day and then take it away the next? No way! If He did that, it would not be Eternal life. If God places an A+ on our paper, we can rest assured that He will never lower it to a B or a C, no matter what mistakes we may make or how many times we may make them. The more I learn about our Great Teacher Jesus Christ, the more I am amazed at His awesomeness! Praise Him!