The following program crashes in the second invocation of AddRef (could be any other method defined by IUnknown or a derived interface) because the "this" pointer points to the IUnknown vtable instead of the object's base, which is assumed by the code.

---
import std.c.windows.com;
import std.c.windows.windows;

class Test : IUnknown {
  int i = 1;
  ULONG AddRef() { assert(i == 1); return 0; }
  ULONG AddRef() { assert(i == 1); return 0; }
  HRESULT AddRef(IID*, void**) { assert(i == 1); return E_FAIL; }
}

void main()
{
  auto t = new Test;
  t.AddRef(); // works
  auto u = cast(IUnknown)t;
  u.AddRef(); // crash in _d_invariant
}
---

Setting this to major severity because it makes defining any COM objects in D impossible on Win64 (and thus many COM APIs are unusable). Tested on DMD 2.063.2