As I rode, the stirrup just didn't feel right. My foot kept knocking around in the stirrup. I ignored it for a while, until I started cantering. My foot almost went through the stirrup. My heel was on the stirrup pad. I immediately halted Razz and decided to ride bareback for the rest of the lesson. I didn't want to risk it.
This had never happened before I started riding Thunder, or in the western saddle. It's possible that someone else needed a wider stirrup-- I lease Razz from a lesson facility, and they may have put a wider stirrup on the saddle.
But does it sound more like the stirrup, or that I need to ride without stirrups more? I ride bareback a lot.. and I usually take my stirrups away for at least 15 minutes of the ride. I even competed in a local fun show's no-stirrup class!