In Android, communications between apps and system services are supported by a transaction-based Inter-Process Communication (IPC) mechanism. Binder, as the cornerstone of this IPC mechanism, separates two communicating parties as client and server. As with any client-server model, the server should not make any assumption on the validity (sanity) of client-side transaction. To our surprise, we find this principle has frequently been overlooked in the implementation of Android system services. In this paper, we demonstrate the prevalence and severity of this vulnerability surface and try to answer why developers keep making this seemingly simple mistake. Specifically, we design and implement BinderCracker, an automatic testing framework that supports parameter-aware fuzzing and has identified more than 100 vulnerabilities in six major versions of Android, including the latest version Android 6.0, Marshmallow. Some of the vulnerabilities have severe security implications, causing privileged code execution or permanent Denial-of-Service (DoS). We analyzed the root causes of these vulnerabilities to find that most of them exist because system service developers only considered exploitations via public APIs. We thus highlight the deficiency of testing only on client-side public APIs and argue for the necessity of testing and protection on the Binder interface - the actual security boundary. Specifically, we discuss the effectiveness and practicality of potential countermeasures, such as precautionary testing and runtime diagnostic.