mrdoob/three.js — pull request #2307 by maximeq (2012-08-10 08:05:25+00:00)
Title: Update docs/api/core/Vector3.html

isZero actually checks the squared length against the "small epsilon".
0.0001 = 0.01² so the function will return true for a length < 0.01.

By the way if there is a good reason for a so limited accuracy it should be given here in the doc (or maybe even in the code).