The importance of APIs in software development, especially web APIs, has increased Developers read documentation, which is available on the internet, and use the corresponding APIs in their products. However, documentation occasionally contains mistakes. Such mistakes can confuse developers or lead to defects that lower the quality of the product. In this paper, we investigate the reliability of web APIs by extracting and comparing OpenAPI specifications from both the documentations and the results of the API calls. Almost half of the documentations are somehow unreliable. Mismatches between documentation and the response can be categorized into four types: 1) Undocumented Keys, 2) Dynamic Keys, 3) Unreturned Keys, and 4) Type Mismatched. This study will help developers design more reliable products.