Answer architecture round 1: one owner keeps a folder's loaded and unloaded names apart, each loaded category is checked against its index entry, addSource returns what it replaced, the manifest cache goes, ErrLoad exits 1, and the docs say what New and Fake now refuse

cmd/fejkdata/main.go

@@ -340,7 +340,7 @@ func (in invocation) write(f *fejkdata.Generator, src input, w io.Writer) error
 	}
 	t, err := f.NewTemplate(src.template)
 	if err != nil {
-		return inputError{err}
+		return templateError(err)
 	}
 	out := bufio.NewWriter(w)
 	for i := 0; i < in.repeat; i++ {
@@ -359,7 +359,7 @@ func (in invocation) recordStream(f *fejkdata.Generator, src input) (func() (*fe
 	if src.record == "" {
 		t, err := f.NewRecordTemplate(src.template)
 		if err != nil {
-			return nil, "", inputError{spacedReference(src.template, err)}
+			return nil, "", templateError(spacedReference(src.template, err))
 		}
 		record = func() (*fejkdata.Record, error) { return t.Fake(), nil }
 	}
@@ -621,6 +621,14 @@ func readStdin(stdin io.Reader) (input, error) {
 	return parseInput(string(raw)), nil
 }
 
+// templateError is misuse unless the data the template reads failed to load.
+func templateError(err error) error {
+	if errors.Is(err, fejkdata.ErrLoad) {
+		return err
+	}
+	return inputError{err}
+}
+
 func fail(stderr io.Writer, err error) int {
 	var te inputError
 	if errors.As(err, &te) {