Over-tightening can cause the plug's metal shell (the hex area which also seals the insulator) to swell, and in turn, puts too much force onto the ceramic insulator or terminal post inside the spark plug. The internals could be damaged which is not visible from the outside.
Under no circumstances should over-torqued spark plugs ever be re-fitted!
A clear sign of over-tightening is when the crushable gasket (if a gasket type plug) is completely flattened. The gasket should not be fully crushed!