Just like you, Heck no, I thought and moved swiftly to H&M to change it.
A yard or less from the entrance I paused. What if the stain was from water, and only looked like a stain at that exact moment when I looked at it. What if I enter this shop, start screaming and make a scene about them selling this piece of clothing with a stain on, and then show it, and... Nothing there.
Few things (on top of my mind) would feel more embarrassing.
So I surreptitiously peaked at the sweater from my spot I was standing at (clearly off from the eyes of the clerks) and discovered, nothing. No stain.
I felt cheated. For two days this stain has forced my new sweater to lay in a bag, cold, alone and tucked away in a corner at the office without the chance for me to wear it.