Problem with Graphics`PolygonUtils`PolygonCombine

A small modification of the original code solves the problem:

Normal@ParametricPlot[{x, x^2 t}, {x, 0, 2}, {t, 0, .1}] /. 
 p : {{__Polygon} ..} :> Graphics`PolygonUtils`PolygonCombine[p]

The new code also works for the initial problem.

You can use Graphics`PolygonUtils`PolygonCombine on all polygons rather than on groups of polygons separately:

pp = ParametricPlot[{x, x^2 t}, {x, 0, 2}, {t, 0, .1}] ;
allpolygons = Join @@ Cases[Normal@pp, {__Polygon}, Infinity];
Graphics[{ EdgeForm[Darker@LightBlue], Opacity[.5, LightBlue],  
  Graphics`PolygonUtils`PolygonCombine[allpolygons]}, Options[pp]]