Tripped at the finish line

(Tripping at the finish line. Trip at the finish line)

An idiom describing a situation where one does great at a subject, until near the end.

Note : To stay true to the meaning, the mistake has to have a great detrimental effect their overall performance.

Jake : Wow, Samantha is an amazing student! She has perfect attendance, she always seems to have the right answers, she turns in her homework everyday, and she aces all of the quizzes.

Tammy : Yeah, but didn't you hear? She completely failed the finals. She totally tripped at the finish line!

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