Today I meant to type
but instead I typed
with the incorrect extra period.
Firefox kicked back the amazingly helpful
which I stared at dumbfounded. All of Firebug's glorious power was seemingly useless against this error. It didn't help that I was working atop a custom Dojo build.
XML descendants internal method called on incompatible Object
Then I flipped back to my editor which thankfully still had the cursor next to the extra period. A simple backspace and I was moving again...