I haven't been able to post in quite a while - far too many things in real life distracting me. But now that I have gotten back to hobby time, I am expanding my system with an I2C interfaced GPIO extender. Everything worked great on the workbench, but refused to work at all once installed. It appeared that the bi-directional buffer chip that I used to connect the Pi to I2C wasn't working, which made no sense.
I broke down and pulled the interface board down and brought it to the workbench. Under the magnifier, the problem was immediately obvious.
Soldering is an art. It takes practice, and I was terribly out of practice when I started this blog (and made this board.) Bottom line, the SDA and SCL lines were shorted by some over-zealous soldering. Since I hadn't used I2C, it was never a problem.