There are question marks (?) when viewing a drawing
The machine that is viewing the drawing needs to have the three SHX files; IDSSIM.SHX, IDSTXT.SHX, and IDSCOM.SHX copied to C:\Program Files\AutoCAD 200x\Fonts folder. These SHX files can be found in any of the FlexCAD product folders (i.e. C:\Idswin\ISO, C:\Idswin\FLO, etc).

Unknown command error when trying to use FlexCAD
Type CONFIG on the command line and hit enter. On the System tab, the check-box option for 'Load acad.lsp with every drawing' needs to be checked [view screenshot]. After this is done, re-open the drawing.