I was unprepared. The three stools I have in the house are not the right height. The potty seat I have (that drops into the real toilet seat) doesn't fit well. The combo of those two means he is uncomfortable and scared of getting himself on the toilet ("I'm going to fall in!"). Rather than reinforce a negative experience, I'm bagging it for the day.
We are now going shopping for the right stool and a better fitting seat (or tools to make the one I have fit better - I'm thinking sticky-back weatherstripping), and diapers. I have pretty much run out.
Next time I do this (in a few days or a week or some weeks), I am going to proudly announce it after the fact. No more public admission of error/failure/unpreparedness for me, no sir.