"Approved: <pwd>" anywhere in the body is letting the post through

I have a moderated list that seems to be working just fine. However, I 
just noticed that some postings that had "approved: <list_pwd>" in the 
body made it directly to the list, requiring no moderator approval.

The strange part is that "approved: <list_pwd>" was the starting line in 
the very first posting (how that happened, no idea!) but all the 
subsequent 'replies' simply quoted it and got through.

I was looking through the list archives but couldn't find any post 
talking about this problem.

Thanks,
Nishi