Today I migrated an IPsec (with IKEv1) site-to-site setup from a pfSense machine to a Debian machine.
Since the pfSense machine was still the Internet gateway for the network, IKE and ESP packets still had to go through it. Now, I recalled something about firewalls not playing too nice with IPsec, so I researched a bit, and I concluded I needed some very specific SNAT rules. (I also realized that IPsec was not really meant for what we're using it for, but over the course of many years enough functionality was kludged together RFCs were written to make it work and industry has adopted it quite widely.)
Well, I created the rules and it didn't work. I spent hours trying to debug the key exchange process, tweaking the strongSwan configuration, capturing packets using the terrible pfSense facilities. At some point I got exhausted, so I deleted all the rules I had added and I began closing everything, including two ssh sessions I had opened to work on the Debian machine. While I was closing the second one, which was running tcpdump, I noticed some unusual (read: different than before) activity. So I reopened a second ssh session and I typed:
ipsec status
...sure enough, deleting all the firewall rules I had created fixed the setup. After some investigation, it turned out they were good enough to make IPsec think it didn't need to use NAT-traversal, but not good enough to make it work without it. I quickly enabled IP forwarding on the Debian machine, created a static route on pfSense, and just like that the site was connected to the other site.
Don't try to outsmart the universe.