@poppa51 I don’t think you did anything wrong, your intentions were good. She was probably defensive because she was embarrassed or something. Don’t let her reaction bother you.
Might I offer another approach you can try if directly doesn’t seem like the correct one? Make yourself look like the idiot. Go up & say “oh your baby is so cute! I love your wrap! I have one too, it’s my favorite way to wear my baby. I have a (brand), what kind is yours? Speaking of wraps…I was wearing it wrong one day! I realized my baby was too low & got so scared. I had to google the correct way to wear baby. Has that ever happened to you?!” & hope she picks up the bait to further the conversation or lets you show her & compare with her wearing her baby. It’s much less confrontational & lets them admit mistakes without getting defensive, because you admitted making mistakes too even if you’re telling a lie in the moment. If you think a direct approach works best, then go with that, but some people don’t take it well.
Now my own story: never place your baby’s carrier on top of the shopping cart where kids sit. Place it in the basket or sit the baby up outside of the carrier or wear them or use a stroller. The extra weight on top of the cart can cause it to tip because it’s too top heavy. It’s not safe & can also damage the car seat. Your baby could get hurt & so could you, trying to catch the baby. Those carts are heavy. I used to mention this to customers in the indirect way above, because I was a cashier & wasn’t about to anger them as I was helping them bag or scanning their stuff. I also have a cousin who responds better to “commiseration” than direct criticism. She loves when I vent about kids to her, hates when she perceives that I’m saying she’s doing something wrong with hers when I’m not. Our family compares us a lot, so she’s always hackles raised around me a bit due to that comparison. So I keep it light, vent mildly, listen to her more, & offer gentle advice to give her reassurance that she’s doing fine. Gentle is better with some people, too much & you’ll push them away.