What are some known hazards with cyclical replication between two MySQL master servers?
Cringe, gag, etc.

5.1 and 5.6 differ by 2 major versions.  Replication is 'guaranteed' to work one direction, not the other.

Don't turn on GTIDs in 5.6; 5.1 will not like it.

Don't use any features new to 5.5 or 5.6.

Simply, don't do it.  Get the 5.1 upgraded ASAP.