In Reply to: Common causes are: use of incorrect antifreeze, posted by Bill R. on December 17, 2000 at 19:28:51:
My guess is that wrong antifreeze causes passage blockage and then overheating. Else it is etching of surfaces and thus channel formation. Either way, milling is again the only solution.
Of course I have also seen overtorquing of head bolts result in crushing and displacement of seal ring of head gasket. If this is the case, then you can usually get away with just replacing the gasket.