The local webserver spawned by the Editor when you select "Build and run" sometimes throws an error and build doesn't load:

An error occured running the Unity content on this page. See your browser's JavaScript console for more info. The error was:Uncaught incorrect header check

To reproduce:
- Install Unity and create a simple scene
- Build and run to WebGL
- If the error doesn't occur right away, refresh the page. Eventually I will get the error if I keep refreshing (maybe one in every four or five refresh). I can make it work again by refreshing also

I believe this is a problem because for casual users it can create the perception that WebGL simply doesn't work. That's what I thought for a while. What I experienced was that the first time after install and I would make a build it worked. But then on all subsequent builds from the same editor instance it seemed to fail.

Behaviour is the same 5.2.3, but the error message is a bit different.

Tested 5.3.1 p1, 5.4 a5, 5.3.0 f4 and 5.3.2 p1.

OS: Windows 10.
Browser: Chrome Version 47.0.2526.80 m