No error, but when pressing the toolbox button the file explorer opens up. 

Command Failure for External Command... System.NullReferenceException: Object reference not set
to an instance of an object

When running an AutoLIST, you get the error;

AutoCAD Error Aborting

INTERNAL ERROR: eNotOpenForWrite message

Command Failure for External Command... ....System.Exception: No routing preference set for...

Layer Notification Warning

Unreconciled new layers exist in the drawing

Unhandled exception.  eAtMaxReaders

When processing a stocklist, you get this message;

Run-time error 94, Invalid use of Null

When trying to insert a hanger, either with layout hangers, single hangers or autohang, you get this error message

It may also put a single random hanger out in space then kick you out of the command

Warning! An error has been detected

Revit could not complete the external command... System.NullReferenceException

Command failure for External Command... Revit could not complete the external command

When trying to break in a rapid pipe/rapid line/rapid main, it will not tee into a pipe.  In the image below, it is not showing the break in triangle.

When trying to insert a sprinkler head, this window pops up;

Pick head insertion point. Pick any point besides endpoint to break into pipe. Can't find file in search path

*Invalid*

When trying to open a .WXF file by double-clicking it, you get this error message;

"The requested operation requires elevation"

The file can be opened if using HydraCALC to open the file by going to the menu [File|Open]

While importing an AutoCAD model into Revit, you get errors listed below;

Revit could not complete external command... System.NullReferenceException

There are identical instances in the same place.. This will result in double counting in schedules. 

When attempting to create a Flexible Connection you get these errors;

Then this;

Could not find file FSC-INSERTDEFLINE.txt

When starting the process to insert a Flexible Sprinkler Connection, you get the error message above;

An error occurred reading the FSC-Selections.txt

Double-clicking your HydraCALC icon makes nothing happen.  You do not see HydraCALC open onscreen nor on your taskbar.  

If you open your task manager and see the app "Hyd_prog.exe" open but not on your taskbar, then the application is stuck in purgatory.

If you see entries here, use END TASK to get rid of them.

Another option may be that it's on the taskbar but not on your screen.  That means it's open but offscreen. 

See this HydraCARD to fix that issue;  HydraCARD 565 only AFTER you've checked on the solution below.

When inserting a VP, the text shows double white text, not white and magenta.  Using the CEL or CUL command still shows the double text