I am in full agreement with Keith, that master/master replication and circular replication spells trouble.  Which is the "authoritative" copy?

 Also, if you have a ring where replication is as follows:

 srv1 -> srv2 -> srv3 -> srv4 -> srv1

(server2 is a slave of server1, etc)

Then if one link in the chain breaks, no new data goes down the line.  For example, if srv3 is down, then changes happen like this:

 srv3->srv4->srv1->srv2 (OK, because srv3 already has the data)

 srv4->srv1->srv2 (Bad, srv3 doesn't get the change from srv4)

srv1->srv2 (Bad, srv3 and srv4 don't get the change from srv1, even though replication on srv4 is working just fine.

 srv2 (bad, no other machines get changes from srv2, even though replication is only broken on one machine).