While it may seem harmless to overstay one day, I would not advise. I spend at least half the year in China and have heard many stories. Overstaying a visa is a crime. While not severe, the penalty is RMB500 per day and you will also probably have to pay for a new visa and be scolded by many. The high side can be worse, it is China and it is their country. If you must stay, get an extension. If you can change your flight plans it would probably be easier. I hope this helps.