Variant: Scoll Mishaps – how and why?

On pg. 140 of the Dungeon Master’s Guide there is a sidebar for handling scroll mishaps. It states:

A creature who tries and fails to cast a spell from a spell  scroll must make a DC 10 Intelligence saving throw. 

While interesting, it leaves a question wide open: how (or why) would a creature fail to cast a spell from a scroll? If the creature can "understand the written language" then the scroll should just work, according to the prior page. Am I missing something?