ExtrudeCrv shows the Solid= option when it is not a valid option.
ExtrudeCrv offers the Solid= option to create a closed solid which works if the curve to be extruded is a closed, planar curve. That makes sense because only for a closed planar curves does the assumption of planar caps work.
However if ExtrudeCrv is started and the selected curve to be extruded is not planar and/or not closed, the Solid= option is still shown in the list of options although the Solid=option is not valid for the previously selected curve to be extruded. ExtrudeCrv then completes even if Solid=Yes is selected. This can be confusing to the user who sees an open extrusion after selecting Solid=Yes.
ExtrudeCrv should only show the the Solid= option when the selected curve to be extruded is planar and closed.