Do the vials look the same? Same color caps? Same labels? If you don’t look at the label, you may give the wrong medicine.
And why was another medicine anywhere near the vax? When dispensing something, you clear your work area of extraneous matter.
Agree. It's unlikely the vials of the vaccine and saline would look identical unless they were manufactured together and shipped together.
Another possibility is Walgreens was instructed to inject the vaccine and saline as a control test.