I've got a problem difficult to analyse: some of my members complain for a long time they cannot post any comment on my blog. But others do comment without any problem ! I changed the antispam plugins several times but the problem still remains.
Last test I did was to desactivate BBQ and one of the blocked commenters says it solves his problem. But it's difficult to see where the problem is exactly - as other people did not have any problem to comment.
I suspect a combination of cache plugin (Quick Cache Pro) + maybe Ajax Comments + BBQ. But it's hard to make tests :-)