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?