Watch and Learn: Inappropriate Speed in a Turn Means Crashing Badly [Video]
This video shows just how bad things can go when approaching a turn with too much speed. And as even the 125cc bikes can easily do 100 km/h or more (60 mph), it's pretty obvious that it's so easy to miscalculate the right speed.
Now, this rider didn't even try to brake before the turn, as he believed rolling off the throttle would be enough. Unfortunately, small-displacement engines don't have enough braking power by themselves, so closing the gas will not slow them as fast as a half-liter engine would.
However, good to see the rider was wearing protective gear and it looks like the fellow was alright, though a bit shaken by the rather strong impact on the guard rail. Things could have been way worse...