mIRC having a security hole with DCC SEND is exactly why some routers decided to filter it. EDIT: Perhaps I'm misremembering this. It seems it might have been due to some bugged port remapping code in the routers.

Nah, I think you're right. Another poster found an issue with Netgear routers doing stateful packet inspection that ALSO failed. So, it sounds like mIRC had a bug, routers tried to apply some protection and also failed.

DCC doesn't work through NAT, as it creates a fresh connection to a random port. That's the entire contents of the DCC send message, the ip/port of the sender and the filename.

I'm pretty sure these Netgear routers were trying to create an automatic port forwarding rule so the DCC connection would work through NAT without any configuration.

Everything I can find points to an issue on some Netgear routers. At least some people say that disabling a specific router feature (stateful packet inspection firewall) would fix it, which seems like damning evidence. Presumably the router didn't actually crash, it just triggered the firewall. Interesting bit of archeological security. The fun part is that nothing has improved.

That's amazing. Thanks for digging that up. Entirely emblematic of early aughts and 90s tech.