Something quite strange is going on:

dattmp={{{0.1,1}},{{"unknown",-0.4},{1,1},{"unknown",-0.5}}};
{ListPlot[dattmp],ListPlot[dattmp,PlotMarkers->Automatic,Epilog->{Circle[{1,1},Scaled[0.03]]}]}
Column[DeleteCases[FullForm[#[[1]]]&/@%,{}|_?ColorQ|PointSize[_]|AbsolutePointSize[_]|AbsoluteThickness[_]|Directive[___],\[Infinity]],Frame->All]

Outputs:

{{{{Point[{{0.1, 1.}, {0.1, 1.}}]}, {Point[{{1., 1.}}]}}}, {}}

{{{{GeometricTransformation[Inset[\[FilledCircle],{0.,0.}],{{{1.,1.}},{{0.1,1.}},{{0.1,1.}}}]},{GeometricTransformation[Inset[\[FilledSquare],{0.,0.}],{{{1.,1.}}}]}}},{}}

The first example outputs 3 points. The second example outputs 4 datapoints. So both are actually wrong?