I very rarely fall down, and regardless of whether or not anyone saw it, I start laughing hysterically. I just can't help it, I guess I just find myself falling down to be really comical. I usually catch myself before I fall completely with my hands.
The only time in recent history I can remember falling completely in public was years ago when I was dancing. We were practicing for a comp, my partner and I, and doing really powerful leaps in a dance called Paso Doble. The dance of the bullfight. My shoe heel caught in my pant leg mid-air and I fell flat on my knees//hands. People came running over asking me if I was alright(which I was, although it hurt), and I was laughing so hard I couldn't even answer beyond nodding my head. xD