Bam Margera took a serious tumble while showing off his skateboarding skills -- and we've got the clip of the exact moment it all went down.
Check it out ... the pro skateboarder whizzes back and forth on the ramps, when suddenly, on one descent, his left leg lags behind ... sending him hurtling towards the ground.
As you can see, the former "Jackass" star's clearly in agony, letting out a loud "F***!" while writhing in pain on the ground. His buddy finds himself in a tough spot here ... watching helplessly, unsure of what to do next or how serious Bam's pain really is.