This is a CentOS/RedHat bug only and can be reproduced with Qt 4 only and only when running the example via remote connection e.g. ssh X forwarding. It doesn't reproduce with Qt 5.1.1.
The attached example can be used to reproduce the issue.
When moving the scroll bar handle quickly, the handle should stop at the boundaries when either top or bottom is reached.
The scroll bar handle goes over the boundaries when moving quickly from up to down or down to up.