If it's only that corner and it's happened multiple times, it's an impossible case. You have to either twist the corner back or take a few pieces out to twist the corner back.I've been playing with my 4x4 for almost a year now and have pretty much figured it out including the OLL and PLL parities... but something strange has happened - I have ONE corner that is in the right place but not properly oriented, and I don't see this anywhere as a known parity. I've tried randomly scrambling dozens of times and cannot get it to solve! Help!
Note: attached images show the fully solved cube with the one corner not oriented; I took two pictures, one from each side to show it's fully solved otherwise.