Very occasionally my chain will come of the small chainwheel on a downshift. It is not at all predictable, except that it always happens when the I'm doing an unloaded shift in preparation for a climb. I don't think it has ever happened under load.
The limit screws seem to be set correctly. Most (>99%) of the time I can shift onto the small chain ring without any problem – both under load and without a load.
I'd appreciate suggestions for what to check or ideas about what could be causing this.