diff --git a/calc_test.go b/calc_test.go
index f37f783..b24696c 100644
--- a/calc_test.go
+++ b/calc_test.go
@@ -72,12 +72,16 @@ func TestCalcFields(t *testing.T) {
 	}
 }
 
-// TestCalcNonNumericIsNaN pins the never-fail rule: a field that doesn't render
-// to a number becomes NaN, which propagates and prints visibly.
+// TestCalcNonNumericIsNaN pins the never-fail rule: a field that sometimes does
+// not render to a number becomes NaN then, which propagates and prints visibly.
 func TestCalcNonNumericIsNaN(t *testing.T) {
-	if got := mustRender(t, engine(1), `{"format":"{calc(x * 2)}","x":"abc"}`); got != "NaN" {
-		t.Fatalf("calc over non-numeric field = %q, want NaN", got)
+	f := engine(1)
+	for i := 0; i < 50; i++ {
+		if got := mustRender(t, f, `{"format":"{calc(x * 2)}","x":["abc","1"]}`); got == "NaN" {
+			return
+		}
 	}
+	t.Fatal("calc over a sometimes non-numeric field never printed NaN in 50 draws")
 }
 
 // TestCalcReproducible pins that a calc over a random operand stays seed-stable.
@@ -218,3 +222,26 @@ func TestCalcHoldIsPerTemplate(t *testing.T) {
 		t.Fatal("the nested template never differed from its parent, want its own draw")
 	}
 }
+
+func TestCalcOverANeverNumericOperandIsRejected(t *testing.T) {
+	for src, want := range map[string]string{
+		`{"format":"{calc(x * 2)}","x":"abc"}`:           `"abc"`,
+		`{"format":"{calc(x * 2)}","x":["a","b"]}`:       `"x"`,
+		`{"format":"{calc(x + y)}","x":"1","y":"{{2}}"}`: `"{2}"`,
+	} {
+		_, err := compile(parse(t, src))
+		if err == nil || !strings.Contains(err.Error(), want) || !strings.Contains(err.Error(), "never a number") {
+			t.Errorf("compile(%s) = %v, want the operand rejected naming %s", src, err, want)
+		}
+	}
+	for _, ok := range []string{
+		`{"format":"{calc(x * 2)}","x":"3"}`,
+		`{"format":"{calc(x * 2)}","x":" 2.5 "}`,
+		`{"format":"{calc(x * 2)}","x":["1","abc"]}`,
+		`{"format":"{calc(x * 2)}","x":"{digits(2)}"}`,
+	} {
+		if _, err := compile(parse(t, ok)); err != nil {
+			t.Errorf("compile(%s) = %v, want it accepted", ok, err)
+		}
+	}
+}
diff --git a/cmd/fejkdata/main_test.go b/cmd/fejkdata/main_test.go
index 86fd3e3..d8bc864 100644
--- a/cmd/fejkdata/main_test.go
+++ b/cmd/fejkdata/main_test.go
@@ -311,4 +311,8 @@ func TestRunNoShippedData(t *testing.T) {
 	if code != 0 || strings.TrimSpace(out) == "" {
 		t.Errorf("run = %d, out=%q", code, out)
 	}
+	code, _, errb = runOut("--no-shipped-data", "sv_SE.person")
+	if code != 2 || !strings.Contains(errb, "--no-shipped-data needs at least one --data-path") {
+		t.Errorf("--no-shipped-data alone = %d, %q, want misuse naming --data-path", code, errb)
+	}
 }
diff --git a/loading_test.go b/loading_test.go
index f7aadfc..9c711f7 100644
--- a/loading_test.go
+++ b/loading_test.go
@@ -181,3 +181,23 @@ func TestHiddenEntriesAreSkipped(t *testing.T) {
 		}
 	}
 }
+
+func TestRepeatProductAlongAPathIsCapped(t *testing.T) {
+	for name, files := range map[string]map[string]string{
+		"nested": {"cat": `{"format":"{a}","repeat":2048,"a":{"format":"{b}","repeat":2048,"b":"x"}}`},
+		"through a reference": {
+			"a": `{"format":"{..b}","repeat":2048}`,
+			"b": `{"format":"x","repeat":2048}`,
+		},
+	} {
+		_, err := New(WithoutShippedData(), WithDataPath(writeData(t, files)))
+		if err == nil || !strings.Contains(err.Error(), "repeat") || !strings.Contains(err.Error(), "1048576") {
+			t.Errorf("%s: New = %v, want the repeat product rejected naming the maximum", name, err)
+		}
+	}
+	if _, err := New(WithoutShippedData(), WithDataPath(writeData(t, map[string]string{
+		"cat": `{"format":"{a}{c}","repeat":1024,"a":{"format":"{b}","repeat":1024,"b":"x"},"c":{"format":"y","repeat":1024}}`,
+	}))); err != nil {
+		t.Errorf("New = %v, want 1024 x 1024 along one path accepted", err)
+	}
+}
diff --git a/shipped_data_test.go b/shipped_data_test.go
index c1d97e3..24aad04 100644
--- a/shipped_data_test.go
+++ b/shipped_data_test.go
@@ -1,6 +1,7 @@
 package fejkdata
 
 import (
+	"errors"
 	"reflect"
 	"slices"
 	"strings"
@@ -55,8 +56,8 @@ func TestUserDataMayReferenceShipped(t *testing.T) {
 
 func TestWithoutShippedDataNeedsASource(t *testing.T) {
 	_, err := New(WithoutShippedData())
-	if err == nil || !strings.Contains(err.Error(), "WithDataPath") {
-		t.Fatalf("New(WithoutShippedData()) = %v, want an error naming WithDataPath", err)
+	if err == nil || !strings.Contains(err.Error(), "WithDataPath") || !errors.Is(err, ErrNoData) {
+		t.Fatalf("New(WithoutShippedData()) = %v, want ErrNoData naming WithDataPath", err)
 	}
 }
 
diff --git a/template_test.go b/template_test.go
index 1f8a59a..1f7f8f4 100644
--- a/template_test.go
+++ b/template_test.go
@@ -121,12 +121,10 @@ func TestAlternationPicksOneField(t *testing.T) {
 	}
 }
 
-func TestWeightZeroNeverChosen(t *testing.T) {
-	f := engine(5)
-	for i := 0; i < 200; i++ {
-		if got := mustRender(t, f, `[{"format":"X","weight":0},"Y"]`); got != "Y" {
-			t.Fatalf("weight-0 variant chosen: %q", got)
-		}
+func TestWeightZeroIsRejected(t *testing.T) {
+	_, err := compile(parse(t, `[{"format":"X","weight":0},"Y"]`))
+	if err == nil || !strings.Contains(err.Error(), "weight 0") || !strings.Contains(err.Error(), "remove") {
+		t.Fatalf("compile(weight 0) = %v, want it rejected naming the fix", err)
 	}
 }