I started getting messages when I tried to update my MacPorts tools. I can’t say when it started, because I don’t do it very often. (Like maybe once a month. Bad me.) But it would bomb out, telling me

Error: checksum (md5/sha1/rmd160) mismatch for port.

Then it sent me down a wrong path, suggesting it was because my ISP’s DNS was serving ads or something. But it turns out the why isn’t important, because the what to do about it was right there in the FAQ:

$ sudo port selfupdate
$ sudo port clean --dist {portname}
$ sudo port install {portname}

All I had to do was clean out the bad portfile and try again.