Onyx HD 500 (eV3, Irvine, CA) is a high-viscosity liquid embolic agent that has recently been approved in the United States as a humanitarian use device for the treatment of wide-neck sidewall intracranial aneurysms. Preliminary evidence suggest that liquid embolic agents can provide improved angiographic results with a lower incidence of recanalization compared to coil embolization. To report unstable Onyx casts and how to deal with them. We report 4 cases of intracranial aneurysms treated with Onyx HD 500 in which, after the aneurysm was successfully obliterated, the Onyx cast was noted to have 1 of 2 types of embolic cast instability. In all 4 cases, an intracranial stent or vascular reconstruction device (VRD) was placed across the Onyx cast at the aneurysm orifice and the cast was stabilized. This series is the first published description of Onyx HD 500 aneurysm cast instability. It is also the first report of using a stent or vascular reconstruction device rescue technique to secure an unstable Onyx cast and represents a new indication for these devices.