8) Once we got everything on the network and server side ready for and usable on IPv6 we discovered that a lot of our client's applications just had no idea what to do with IPv6 connections. Many PHP applications broke because they expected $_SERVER['REMOTE_ADDR'] to fit within 15 characters at most. Databases had to have their columns widened (if they were storing the address as text), or functionality had to be rewritten if they were storing IPs as 32 bit integers. Web server log analyzers claimed that the log was "corrupted" if it had an IPv6 address in it. Lots and lots of application logic just wasn't IPv6 aware at all, and either had serious cosmetic problems with displaying IPv6 addresses, or simply didn't work when an IPv6 address was encountered.
Just imagine what it will be like if the idea of sticking a decimal point into 32 bit AS numbers ends up getting deployed. --Michael Dillon