This article presents an experimental investigation on the short-circuit (SC) instability of several commercially available cascode gallium nitride (GaN) high-electron-mobility transistors (HEMTs). In the SC test, self-sustained oscillation is observed during the SC transient. The SC oscillation features some unique characteristics. The gate resistor shows very weak damping effect on the SC oscillation. The SC oscillation thereby cannot be suppressed by utilizing a large gate resistor. With the increase in the dc-bus voltage, the SC oscillation greatly amplifies and becomes more unstable. When the dc-bus voltage reaches 200-250 V, catastrophic failure occurs. In the SC test, two distinct failure modes, which are related to the failure of low-voltage (LV) MOSFET and depletion-mode HEMTs (DHEMTs), are identified. Based on the experimental evidence and simulation study, the root causes of the failure are clarified in the end.