I was upgrading a port on a FreeBSD box, like a gentleman:

# cd /usr/ports/misc/p5-Chatbot-Eliza
# make

And got this:

/!\ ERROR: /!\
Ports Collection support for your FreeBSD version has
ended, and no ports are guaranteed to build on this 
system. Please upgrade to a supported release.  
No support will be provided if you silence this message 
by defining ALLOW_UNSUPPORTED_SYSTEM.  
make[1]: stopped in /usr/ports/misc/p5-Chatbot-Eliza
*** Error code 1

But wait a minute, aren’t I running the latest version?

# uname -rs
==> FreeBSD 11.1-RELEASE-p6

Just to confirm though:

# freebsd-version
==> 11.0-RELEASE-p16

Ah hah! When there’s a mismatch like this, it usually indicates an incomplete upgrade:

# freebsd-update fetch
==> You have a partially completed upgrade pending
==> Run '/usr/sbin/freebsd-update install' first.

Finishing the update install got me the correct version, and I was able to build the port.

==> ELIZA: And how does that make you feel?