Hi there,
It sure sounds like that person was being sneaky, but it's tough to know for sure. I would say the best way to avoid these problems is to have a few folks monitoring the silent auction area towards the end. If they were to see anything like that and realize what was going on, they could politely ask the person to finish up. It would take a good deal of diplomacy, however. These parents are there to support your auction. You don't want anyone manipulating the system, but you don't want to "police" the event too strenuously, either.
Sorry this happened. Sounds like it was disappointing for some of the other bidders.
Our auction was this past weekend. There was a situation that came up that had people talking. One of the items up for bid had several people bidding throughout the event. Right at the end with about two minutes left, a parent wrote her name and then proceeded to have a conversation with her daughter about whether or not she really wanted the item, stalling before writing in her amount, holding the pen in her hand until the last few seconds. What is the best way to handle a situation like this? I'm not sure what she did was wrong, but it surely was poor auction etiquette.