strange ConnLimit behaviour

Hi,

I'd like to set a global limit for IIS 6 instances and I'm faced with
quite wierd behaviour.
There's no 403 (too many users) status messages in the access logs and
HTTP.sys log
report that 503 error code was returned to clients when limit was
reached.

Moreover, appliaction stopped responding and only turning limts off
completely
persueded it to go on.

I can see there is some condratictory documentation on msdn on which
error code should
be returned to clients and what is the expected server behaviour.

Could you please enlight me on the subject ?

Cheers,