pfSense

When one configures IPv6 NPt (network prefix translation) to use a public prefix that does overlap with the interface's own address/prefix, IPv6 suddenly stops working in surprisingly mysterious and hard-to-troubleshoot ways.
While this misconfiguration is clearly user error, it would have saved me about 6-7hrs of troubleshooting if this were either disallowed or a warning of some sort were emitted. I very much doubt I'm the only person who will ever make this mistake.

(FYI: it's hard to troubleshoot because no packets get blocked, no [visible] rule counters get incremented, the packets show up in tcpdump, and yet everything fails because NDP adjacency now fails! So there are no hints to the poor sap troubleshooting this until they finally try "pfctl -d" in desperation. I'm not sure how the average user without deep IPv6 knowledge would ever find this.)