In the end of Skyfall, M says that she did get one thing right. By my understanding she meant Bond.
But, what exactly did she mean in that dialog?
She meant that she was right in trusting Bond to do the job, even if he failed the tests, and failed in the other choices.
M accepts her failure a little while before they get prepared for Silva to come.
M: I f**ked this up, didn't I?
James: No. You did your job.
She was right in one choice and James Bond proved her right. That's all she meant by that dialogue.
I like the other answer, but it won't let me comment as I don't have high enough reputation, so I will leave this as an alternative. Certainly, she was right about Bond both specifically (failing tests) and generally, but she was right about one other thing too. She was right about what she said in his obituary "An exemplar of British fortitude." He doggedly continued to follow and fight right to the end, and that fortitude saved M directly. Further evidence for this interpretation is the gift of the porcelain British bulldog, which is also a symbol of British fortitude generally and specifically (survival of the explosion).