This error appears immediately when trying to run Create Stocklist, the dialog box never appears. 

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

Install or Update appears to hang.

When opening Valve Selector received the following message: Initialization Failed
Application failed to initialize.

This error may start out as "Could not load default information for the Pipe Model settings"

But will then turn into an AutoCAD fatal error.

When selecting the Navis Toolkit then the SETUP button, you get the error;

AutoCAD Error Aborting

Fatal Error: Unhandled Access Violation 

None

Command Line in AutoCAD is missing

While trying to calculate a job, an error message pops up saying the following

"Error in Calc Engine"

Error in processing data of calculation;

-If this is a grid, uncheck 'Autopeak' and try again or,

-Confirm the node you set as the calculation source node exists in your input or,

- Check for incorrect values in your densities, areas or pressures

Unhandled Exception has occurred in a component in your application.

One of the identified items was in an invalid format

During running your calculation, and after receiving the summary regarding your safety margin, you get a similar error.

You'll then likely get these errors, up to 3 times.

Be aware these windows may be BEHIND the calc window, so move the calc window out of the way if it looks like it has crashed or is hung; 

When trying to insert a riser nipple/vertical pipe, you get a FATAL ERROR:  Unhandled Access Violation. 592A00h or 8B2A00h

List of available hangers is blank or very short...

Missing Basic Adjustable Ring Hanger

Missing Hanger Families

Trying run a calculation as stand-alone, or via AutoCAD/AutoCALC or Revit/AutoCALC, you get this error message;

Run-time error 339:  Runtime error 339  Runtime 339
Component 'MSCOMCTL.OCX' or one of its dependencies not correctly registered: a file is missing or invalid

It's likely a job without a pump works, but one with a pump crashes
 

During the install of HydraCAD V56/57/58, the user is greeted with the message below.  Following the both sets of instructions of opening and closing AutoCAD, and running a repair of AutoCAD do not make a difference.  You'll still get the error message.

V56 = R23.X
V57 = R24.X
V58 = R25.X

V57 install will say;

Neither AutoCAD 2021, 2022, 2023 nor 2024 found (missing registry key:

HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R24.X).

Please install it before re-running this install

The following sprinkler tags are not loaded ...

Unable to import from the latest toolbox. I check all of the boxes in the “update” window but the “continue “, ”help”, and “cancel” options do not appear at the bottom.

When trying to print final, the dialog box HydraVIEW, Run-time error 9, Subscript out of range

During the install of HydraCAD the following message(s) pop up;

"Unable to Create HydraCAD Shortcut" "Unable to find registry,"

"HKEY_CLASSES_ROOT\AutoCAD.Drawing.#\protocol\StdFileEditing\server"

The version of AutoCAD will determine the number following the word 'Drawing' in the registry path.

AutoCAD 2022 = 24.1

AutoCAD 2021 = 24.0

AutoCAD 2020 = 23.1

AutoCAD 2019 = 23.0

AutoCAD 2018 = 22.0

"Unable to find default profile" "The default profile 'HydraCADVersion56' 'HydraCADVersion57' does not exist.

When trying to edit the company information by going to the menu [Utilities|Alter Company Information], you get the error saying, "Error trying to edit company information", Please get a copy of {\...\Company1.dat} onto this computer".

Missing first outlet