Possibly a folder marked as "hidden" in Autocad.

HydraCAD Critical Error... Visual Lisp not available

No message. Double-clicking a DWG file to launch AutoCAD opens drawing but also starts a new drawing or brings up the Start Menu.

When opening HydraCAD, you get this error message or something similar;

AutoCAD Warning

The following shortcuts in the search were invalid and ignored: 

After clicking OK, HydraCAD will open up, albeit very slowly, and almost every step will take an significant amount of time, for example 45 seconds to start a new drawing or to size a pipe

When opening HydraCAD you get the error;

Exception in hcadmainbase.arx ARX Command

Unhandled Exception c0000027 (c0000027h) at address aca746e7h

When trying to open HydraCAD after a recent install, you get this error or one similar to it;

"Missing shortcut", Windows is searching for acad.exe.  To locate the file yourself, click Browse.

AND

Problem with Shortcut - The item 'acad.exe' that this shortcut refers to has been changed or moved, so this shortcut will no longer work properly.

Nearest match based on size, date, and type:

Do you want to fix this shortcut to point to this target or do you just want to delete it?

When opening HydraCAD and trying to open a drawing or a template, you get the error message; 

Also happened when layers were isolated trying to burst objects

'unrecognized version and cannot be read'

When opening HydraCAD, you get this error

Unhandled Exception...DAO32Server...

Layer Notification Warning

Unreconciled new layers exist in the drawing

Unhandled exception.  eAtMaxReaders

When opening HydraCAD, you get the error message saying, "The Ribbon does not have any tabs or panels currently loaded"