Should x264 be non-deterministic when '--non-deterministic' isn't used?

[Selur]

Over at videohelp, there is someone you get's non deterministic output even when he isn't using '--non-deterministic'.

Some folks say that:
a. x264 is always non-deterministic as soon as multithreading is used.
b. x264 is always non-deterministic as soon as VBV is used.
c. x264 is always non-deterministic as soon as VBV&multithreading are used together.
I though that x264 should be deterministic as long as '--non-deterministic' isn't used and if it isn't there is a bug?
Also when a., b. or c. is correct, what is the use of '--non-deterministic' ?

Can someone knowing the code answer this?


Cu Selur