Programmers often consult an online Q&A forum such as Stack Over ow to learn new APIs. This paper presents an empirical study on the prevalence and severity of API misuse on Stack Over ow. To reduce manual assessment e ort, we design ExampleCheck, an API usage mining framework that extracts patterns from over 380K Java repositories on GitHub and subsequently reports potential API usage violations in Stack Over ow posts. We analyze 217,818 Stack Over ow posts using ExampleCheck and nd that 31% may have potential API usage violations that could produce unexpected behavior such as program crashes and resource leaks. Such API misuse is caused by three main reasons—missing control constructs, missing or incorrect order of API calls, and incorrect guard conditions. Even the posts that are accepted as correct answers or upvoted by other programmers are not necessarily more reliable than other posts in terms of API misuse. This study result calls for a new approach to augment Stack Over ow with alternative API usage details that are not typically shown in curated examples.