I've reorganized the tunnel detail page a little bit, adding section titles. I could bold the differing digits in the /64s, but if people don't catch the difference as it is now, not sure how much more useful bolding it would be.
I guess we could have a banner parade across the screen: "The /64s are different *elephant trumpets*" and the like, but I think that would be a bit overkill. ;-)
I was thinking something more like making it a different color or something, or sectioning it off into a separate box ... something like that. Or perhaps just a little parenthesized red text under the client /64 or something saying don't use this for your IPv6 addresses or something similar.
The problem is that except for one digit, the addresses are identical, and that's can be hard to catch visually. It makes it easy to do things like cut and paste the wrong line and things like that.
EDIT: In fact, I admit that when I set up the tunnel, I somehow "visually filtered" the routeable /64 on that page as being a repeat of the 6in4 tunnel address, and presumed that we weren't assigned a routable network until we requested a /48. I just didn't look close enough at the headings, or examine the addresses close enough to see that they were actually different IPs.
EDIT2: Looking at the page, I see that they actually are in their own sections. Not sure if they were back when I set up my tunnel.