Two of the most common causes of this are:

1. An error, inconsistency, or unexpected
   mapping in the DNS registration
   
   This happens frequently: your configuration restricts
   access to host.domain.com, but you can’t get
   in from that host. The usual reason for this is that host.domain.com is actually an alias for
   another name, and when Apache performs the
   address-to-name lookup it’s getting the real
   name, not host.domain.com. You can verify
   this by checking the reverse lookup yourself. The easiest
   way to work around it is to specify the correct host name
   in your configuration.
2. Inadequate checking and verification in your
   configuration of Apache
   
   If you intend to perform access checking and
   restriction based upon the client’s host or domain
   name, you really need to configure Apache to
   double-check the origin information it’s supplied. You
   do this by adding the -DMAXIMUM_DNS clause
   to the EXTRA_CFLAGS definition in your
   Configuration file. For example:

   EXTRA_CFLAGS=-DMAXIMUM_DNS

   This will cause Apache to be very paranoid about
   making sure a particular host address is
   really assigned to the name it claims to be.
   However, note that this can incur a significant
   performance penalty, because of all the name
   resolution requests being sent to a nameserver.