i think the object is working correct (wrong in reference): http://reference.wolfram.com/mathematica/ref/ArcCos.html

If you think for a moment about what acos() is doing, it's clear that the output range can only be a half cycle (i.e., 180° or π) when working with reals (i.e., not complex values).

Mathematically, there is a certain logic in declaring the range [-π/2 .. π/2]. But the standard digital-arithmetic implementation shifts the range to [0 .. π] (see man acos(3)).

The Max objects for all calculations are simply wrappers around the standard C math libraries, so if in doubt, that's where to look.

"If you think for a moment"

Hehe yes indeed. I might have if my poor brain wasn't so deadline stressed right now!

So yes, the bug's in the reference, not the object.