User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:6.0.2) Gecko/20100101 Firefox/6.0.2

If you connect with ssh -X to a machine that has IPv6 disabled in YaST, it will not forward X.

This appears in /var/log/messages:

error: Failed to allocate internet-domain X11 display socket.

or, in DEBUG3 LogLevel:
debug3: sock_set_v6only: set socket 8 IPV6_V6ONLY
debug2: bind port 6010: Cannot assign requested address
debug3: sock_set_v6only: set socket 8 IPV6_V6ONLY
debug2: bind port 6011: Cannot assign requested address 

Reproducible: Always

Steps to Reproduce:
1. Disable IPv6 with YaST/Network Settings
2. ssh -X localhost
3. run xclock
4. error: Cannot open DISPLAY
Actual Results:  
Cannot run graphical applications when connecting with ssh -X to remote machines that have IPv6 disabled

Expected Results:  
X11 being forwarded