Cute guy at school tells girl: "hey" send me a snapchat pic of..."

Girl: "ummm... but I don't want anyone else to see the picture but you..."

Guy: "DONT WORRY! Snapchat doesn't allow saving of pics, it will disappear from my phone after a few seconds."

Girl: "oh, okay I guess..." *sends pic* That pic is later sent back to her by her friend who got it from someone, who got it from someone else who found it on some website somewhere.

There are apps you can download on your phone that save every video and screenshot you view on snapchat.