Vyper is a pythonic Smart Contract Language for the ethereum virtual machine. In affected versions, the return of .returns_int128() is not validated to fall within the bounds of int128. This issue can result in a misinterpretation of the integer value and lead to incorrect behavior. As of v0.3.0, .returns_int128() is validated in simple expressions, but not complex expressions. Users are advised to upgrade. There is no known workaround for this issue.