Could not determine size order...

When hitting Save, Refresh Drawing or Calculate Results, user gets a Run-time 9 error. AKA Subscript out of Range

When opening AutoCAD, you get an error saying "a software problem has AutoCAD to close unexpectedly"

Upon reviewing the file it's crashed on, it shows HCAD_MAKEFIGHANGERLIST as the culprit

When trying to create a new Info Sheet, you're greeted with the error; 

Error during load of .INF data, mismatched file versus form

During the install of HydraCAD, you get an error, "Setup Needs the Next Disk, Please insert disk 1 that contains the file data2cab"

Runtime Error 3159: not a valid bookmarkFile has input errors, user gets Runtime error 3159, HydraLIST closes and that file cannot be reopened. runtime error 3159

Clicking an ID brings this window and HydraLIST closes.

Run-time error '3159':  Not a valid bookmark

HydraLIST process freezes at Processing Cover Page Report - Optimizing pipe

Or freezes at processing pipe sketches

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.  Scroll into BACKGROUND PROCESSES as well as they may not show up in "Apps"

Another possibility 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.

Revit could not complete the external command... Curve length is too small for Revit's tolerance