When I first started doing microstrip RF distributed elements like UHF notch filters I was really confused by the results I was getting from my $400 pocketVNA. The microstrip filters worked when I had them in my physical RF pipeline but the S11 I was measuring was crazy. Eventually I gave up.

One day while complaining on IRC someone suggested I let the VNA warm up before doing the calibration routine. I'd just been doing it right after turning it on because naturally, that's what was required first. I waited 15 minutes and then did the calibration. Suddenly the S11 plots actually matched the behavior and I could compare directly against the sonnet fullwave sim design instead of just "it seems to work".

With only several dollars worth of high-precision PCB specifications, you two can save multiple fractions of a cent worth of passive components!

There are applications where this can be useful, though. Especially when PCB production quirks can be more repeatable than component soldering, which can be affected by ambient temperature, humidity, solder paste age, and a bunch of other factors.