Hi,
I don't know if you could really call this a bug or maybe it's working as intended and I'm doing it wrong... (sorry)
If you get sufficiently close and intimate with a mesh you can't pick it.
Here's a playground:
http://www.babylonjs-playground.com/#1KNCOA
The important parameter to look at is the Z coordinate of the camera.
If I set it to -1.06 then I can successfully pick the front face of the box and distance to the box is 0.01. (I'm printing pickResult in the console)
If I set it to -1.04 then I can only pick the back face of the box yet I can still see the front face.
If I set it to more than -1 then I can't see the box so I can't pick it, that's normal.
It's as If the ray's origin wasn't exacly in the camera's viewframe but 1 unit length in front of it.