This bachelor thesis investigates the consequences of the lax implementation of the standards by evaluating various parser on diverse inputs. It generates and optimizes invalid JSON by using well-known techniques of software testing. Moreover, the thesis highlights that over 50% of the analyzed parsers are error-prone to wrong JSON Arrays and JSON Objects. Whereas the structure of most illegally parsed inputs is already known, it also points out some new designs. Furthermore, it demonstrates that the problem is not restricted to specific programming languages but is still a widely-spread problem.