If you say that someone overreaches themselves, you mean that they fail at something because they are trying to do more than they are able to.
He overreached himself and lost much of his fortune. [VERB pronoun-reflexive]
The company had overreached itself and made unwise investments. [VERB pronoun-reflexive]
The people who sustain the worst losses are usually those who overreach. [VERB]
Synonyms: trick, cheat, sting [informal], deceive