And now for something completely different (thank God) …
Okay. Mostly my bad.
Clearly the template editing processes is doing some interpreting or compling of the template as it is saved. So, for example, you cannot save a template that does not have a [Blogger] or [/Blogger] tag, or lacks that section altogether. Which is what was happening (I left off the closing tag).
Only mostly my fault because, well, jeez, folks, interpreters/compilers have been giving meaningful error messages for, well, decades now. You don’t just say, “Huh, doesn’t fit the format, so lah-dee-dah, we’ll just pretend he never hit save.”
Now I just have to keep tweaking it so that it all fits on the screen and so that it loads in something less than five minutes. I have to do something about my nested tables, clearly.