A computer that expects ipv6 tries to talk ipv6 to a router that does not speak ipv6. The router goes wtf and throws a glob of goop at it. Catastrophic failure is thus averted.
Madness string 2290
It's always going to be dangerous no matter what. People don't like to hear that they're wrong or that they're doing bad things. But there's always going to be someone who needs help, and if she's serious about it, it's not something that's going to end.