diff --git a/media/gpu/windows/d3d12_video_processor_wrapper.cc b/media/gpu/windows/d3d12_video_processor_wrapper.cc
--- a/media/gpu/windows/d3d12_video_processor_wrapper.cc
+++ b/media/gpu/windows/d3d12_video_processor_wrapper.cc
@@ -147,6 +147,18 @@ D3D12FenceAndValue D3D12VideoProcessorWrapper::ProcessFrames(
     output_stream_desc_ = output_stream_desc;
   }

+  // The previous ProcessFrames() submission is normally drained transitively
+  // via D3D12VideoEncoderWrapper::Encode() -> SignalAndWaitCPU(). If the codec
+  // delegate's EncodeImpl() bails out before reaching that wait, the previous
+  // command list may still be executing on the GPU when we get back here.
+  // Resetting an allocator backing an in-flight list is D3D12 UB, so block
+  // until the last signalled value has been reached on the GPU.
+  if (fence_->GetCompletedValue() < fence_->Value() &&
+      !fence_->WaitCPU(fence_->Value()).is_ok()) {
+    DLOG(ERROR) << "WaitCPU on previous video-process fence failed";
+    return {};
+  }
+
   hr = command_allocator_->Reset();
   if (FAILED(hr)) {
     DLOG(ERROR) << "Reset video process command allocator failed:"