Embedded devices can be exposed to a wide range of attacks. Some classes of attacks can be mitigated using security features or dedicated countermeasures. Examples include Trusted Execution Environments, and masking countermeasures against physical side-channel attacks. However, a system that incorporates such secure components is not automatically a secure system. Partial Key Overwrite attacks are one class of attacks that specifically target the interface between different components of the security system. These attacks may allow an adversary to extract otherwise protected cryptographic keys through careful manipulation of memory-mapped registers. So far this powerful class of attacks has received little attention in the academic literature. In this work, we provide an overview of known Partial Key Overwrite vulnerabilities and how they were used in real-world attacks. Additionally, we evaluated 31 common microcontrollers and embedded microprocessors from eleven distinct vendors and detail our findings. Based on a first high-level evaluation we selected 15 SoCs and performed an in-depth evaluation. This evaluation revealed that at least eight of these SoCs are vulnerable to partial key overwrite attacks.