When a book is like:

Ana was a cheerful 10 year old girl who loved tacos. She went to school every day with a smile plastered on her lips.

Ana went to school one day and did the same as always. She greeted her friend, Maggie.

"Hey Maggie, how are you?" Ana said.

Maggie was also very cheerful. She was intelligent, and she loved tacos. She hated school though. Maggie was Ana's best friend.

Ana expected Maggie to answer with a sweet "I'm okay", but she didn't. Instead, Maggie grabbed a knife and stabbed Ana on the stomach.

Well then... That's a huge plot twist. And nothing makes sense. First she is cheerful and her best friend and then an assassin. .-. ._.

Don't go all Killua. (HxH)

Or some stories where Harry Styles is really a vampire who likes to capture 12 to 17 year old girls just for fun.

Like wut.

PLOT TWISTS ARE GOOD, BOT NOT SUPER MEGA HUGE PLOT TWISTS.

NO.