What happened?

[refnrev]

Last night I had a first and I am not sure exactly what or how it happened. First game of the V match. Both teams using the L but not to serve. I'm U. 25 purple goes to the proper area to sub in for 15. The only problem was that according to my lineup card 25 is already in the game. I call an official TO and check with the book. No one has 15 in the game and no one knew when she subbed in. She didn't sub in through me. L Tracker didn't have it, scorer didn't have it. No one. I just had 25 wanting to sub in and according to all of us 25 was in the game. She (15) didn't come in during a TO so we had to assume she came in incorrectly on a libero substitution but no one had any idea when. Since no one had any idea when she came in no one knew exactly how to record it or to correct it or penalize it. At the conclusion of the game I told the coach to make sure everyone subbed in and to do the "dispsy doodle" as they call it around here just as if L were serving next game. She had them do that the rest of the game and no more problems.
OK any idea what happened? I know we had an illegal substitution somewhere.... but no one knew when, where, or how? Any ideas on this. What should have happened and what should have been done?