I think Peter just pointed out the answer: AA jitter.

Here come the docs (subject: "jitter", actually about NOT
using jitter for animations):

"Jitter is a very small amount of random ray perturbation
designed to diffuse tiny !!!aliasing!!! errors that might
not otherwise totally disappear, even with intense
anti-aliasing. By randomizing the placement of erroneous
pixels, the error becomes less noticeable to the human eye,
because the eye and mind are naturally inclined to look for
regular patterns rather than random distortions."

Conclusion: there is a random function in AA.
the docs continue:
"For this reason, we should always set jitter to off in area
lights and !!!anti-aliasing!!! options when preparing a
scene for an animation."

Conclusion: beside area lights, jitter is also used in AA.
Problem solved?

Thanks for the many replies

Julius Klatte

>I think it's AA jitter. Set jitter to 0.0 (+j0.0)
regardless of your
>AA setting (turn it off as well). Maybe jitter randomises
the ray
>position within the bounds of the current pixel even though
>anti-aliasing is turned on. Is the opposite explicitly
stated
>anywhere?

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