How can you use a condom incorrectly?

Ali • 27 years young. 4th Grade Teacher 🍎📚🖍
Serious question, they say condoms are like 85% effective because people don't use them correctly but if used correctly they're more like 98% effective at preventing pregnancy. 
I just don't understand how someone can use a condom incorrectly.. you just put it on the penis and that's that! Am I not thinking outside of the box or what?
Thank you ladies! That did help, I guess I just wasn't really thinking of the little things!