If your front wheel hits something big enough to stop it (a curb will suffice), you may be suddenly ejected straight over the handlebars, head first until/if you get your arms up. The front wheel does stop, but the front wheel converts all the kinetic energy that was propelling the bike forward into the frame and rear wheel rotating up and over the now-stopped front wheel, including lifting you up and off the seat & pedals. What you think might happen immediately after that determines whether you wear a helmet or not. A sideways fall - even doing circles in a parking lot - can result in your head being the first body part that hits the pavement.

When I was a teenager, I got "ejected" off my 10-speed once, went clean over the handlebars and landed upright on my feet gymnastics dismount style, with the bike summersaulting behind me. All that was missing was the "ta-da" sound!