TWO SYMPTOMS:

Drawing fittings and pipe look disjointed, like that time an alien team of scientists were performing out-of-phase medical experiments on the Voyager crew.

Or, on the way into Revit, you get this error;

Error (must be addressed in order to continue)

Revit has experienced an unexpected error.  The operation has been cancelled to protect your project from corruption

Deleted element

When reporting the error, you may see this;

Hashtable head_table, Hashtable FLXpipeTable, List '1 listFtgs, flexpipes PipeTypeInfo ptInfo