If you do it often, the airline will get unhappy with you (e.g. cancel your frequent flier account, or bill your travel agent/corporate travel department if they buy the tickets for you).

Many years ago (remember the era of "Ticket for sale: Boston to Los Angeles, Dec. 28, male name"? That many) I was doing hidden-city, and the departure gate for the connection was right across from the arrival gate. As I walked past it, I heard someone talking with the GA about getting on as a standby, because catching that flight was very important to him. I gave him my boarding pass.