I can admit that the stock solution that caused my failure was in a small (~360ml) bottle that was probably squeezed too much and, even so, still had some air space. I did think it was odd when I clip tested it that the clip didn't darken as much as usual, but figured it must be fine if it was darkening at all. My mistake, my loss, lesson learned. I can't speculate if this was regular oxidation or the "XTOL surprise," but either way, I've not experienced any other developer lose its efficacy this abruptly.
I had the remaining stock of this XTOL, which I mixed in February, stored in a two liter soft drink bottle filled to capacity and stored in the dark. I decanted it last night to smaller bottles (glass and thick plastic). I developed a sheet of T-Max 400 (4x5) in both this remaining XTOL (1+1) and HC-110 (E). I wanted to see not only if the XTOL was still good (it is), but also how the two responded to constant rotary agitation. If anyone's interested, I'll post some examples later. At a glance, though, for all the talk about HC-110 building density to quickly under constant agitation, I'm just not seeing it. I might blow through this remaining XTOL as quickly as possible and then go back to bulletproof HC-110.