There are two cases that create the 'outside world' error. The first is an invalid brush, almost always created by a vertix manipulation gone wrong. The coordinates listed in the error are very important in diagnosing the error. If any of the coordinates are -9000 or 9000, then the brush is indeed invalid, and most likely needs replacing completely.
The other case is when the brush actually is outside the world. The editor being just an interface to some objects stored in 3D math, doesn't really care if your brushes are overlapping or even completely outside the allowable world coordinates of +/- 4096. The fix here is pretty simple, just move the brush back inside the allowable region. For sanity sake, don't put brushes right up on the 4096 border. I would make the limit 4000 exactly, so there isn't as much room for an 'accident' while editing.