No error, but in inserted Valve is misaligned or not in the desired direction. There may also be some other visual issues. 

When using Rapid Pipe/Rapid Line/Rapid Main, you do not see the tooltip which indicates your future pipe lengths

Bad location of K-Factor

When trying to run a calc for the system, this error appears: 

After using the Spacing and Layout tool, the pipes and sprinklers are modeled but are not connected properly to each other. 

Compile error in hidden module: CollectInvData

No error, but when opening the file menu and opening Revit, the program freezes or stutters anywhere between 10 seconds - 1 minute. 

Processing gets stuck / freezes at optimization for pipe type(s)

Unable to load defaults during HydraCALC Setup. Occasionally will give an error, but most commonly nothing will be filled in. 

Inserted sprinkler is disconnected / malfunctioning & the branchline pipe is missing

Revit could not complete the external command... No work plane set in current view

Trying to run a calculation, you get the error;

Could not Initiate Process called for by: (name of the problem file), ie. HydCalc.exe or Miscfunc.exe

Revit could not complete the external commad... Method not found

While attempting to raise to 3D, you get this error message;

Exception Occured at AddURLToHyperlinks::HydraHeads

The drawing will still raise to 3D but you may not have sprinkler heads visible.

(TYPE_E_CANTLOADLIBRARY)

679327F1-F31E-488D-8FBC-124C4F6F588E

DAO32ServerLib.DaoComObj

When trying to insert a Hydraulic demand block (Compact/Standard/Expanded) (IHD command) and after selecting the .WXF file to use;

You get the error message;

Error: bad argument type: numberp: DATAFILE

And the command kicks you out.

After setting up a fitting in the Parts Tree then trying to Insert Fitting, you get the Unhandled Exception error below

eLockViolation

When starting the AutoCALC process, you get the error; Unhandled exception has occurred in a componenet in your application

eOnLockedLayer

Pretty Sprinklers does not seem to work with sidewall heads.

Glyphs used to indicate the component type (Line/Main/Armover/Underground) that riser nipples are to be listed as remain on drawing even after command is closed

Component TABCTL32.OCX or one of its dependencies no correctly registered: a file is missing or invalid Unhandled Exception n/a