The only issue that I had...I used 50/50 and 100% cotton t-shirts...was that my iron was too hot when I ironed the transfers on the red shirts and the red bled through. I ruined several shirts. With other transfers...you want the iron as hot as it will go but the iron can be too hot with Amy's.
Beware of red shirts! There was another family at Disney when I was there and the red bled on her shirts as well.