diff --git a/docs/active-work.md b/docs/active-work.md
index 56faf09..ff4075e 100644
--- a/docs/active-work.md
+++ b/docs/active-work.md
@@ -290,8 +290,9 @@ from #171 and #215.
   the shell needs them.
 - **Conformance vectors live in `tests/common/sigint_conformance.rs`**
   and are consumed by BOTH validators, so the two copies cannot drift
-  while each still reports "45 cases". Every stub emits a **sentinel on
-  stderr**, which is what separates `ValidatedError` from `Boundary` —
+  while each still reports "45 cases". The **branch-discriminating** cases emit a
+  sentinel on stderr, which is what separates `ValidatedError` from
+  `Boundary` —
   they share exit 2, so comparing codes alone let a validator that
   accepted every status 2 pass the whole matrix. An `Outcome` enum
   (Safe / ValidatedIgnored / ValidatedError / Boundary) is asserted
@@ -319,9 +320,15 @@ from #171 and #215.
   `gate_validates_the_whole_shared_conformance_set`; token-to-stderr
   fails `sigint_helper_reports_safe_…`. An earlier entry said the
   status-2 mutation was caught only by the dedicated row — that was
-  true of the pre-sentinel matrix and is **superseded**: every stub now
-  emits a sentinel on stderr, so the matrix can see which branch
-  produced the exit 2.
+  true of the pre-sentinel matrix and is **superseded**: the
+  branch-discriminating cases now carry a sentinel, so the matrix can
+  see which branch produced the exit 2.
+- **X3 and X4 deliberately carry their OWN stderr payloads**, not the
+  sentinel — X3 the canonical ignored wording with no token, X4 noise —
+  which is what makes them distinct inputs. `shared_cases()` asserts
+  uniqueness over `(status, stdout, stderr)`, so the earlier
+  45-entries-over-43-inputs collapse cannot recur silently; reverting
+  either payload now fails by name.
 - **Two PRE-EXISTING crdt-only failures found while gating properly:**
   `m4_24_bare_string_glob_stays_relative` and
   `m4_24_d3_fallback_base_is_the_smallest_attachment_dir`. They
diff --git a/tests/common/sigint_conformance.rs b/tests/common/sigint_conformance.rs
index 04644b5..f7f2d68 100644
--- a/tests/common/sigint_conformance.rs
+++ b/tests/common/sigint_conformance.rs
@@ -29,8 +29,16 @@ pub const TOKEN_SAFE: &[u8] = b"pmacs-sigint-v1:safe";
 pub const TOKEN_IGNORED: &[u8] = b"pmacs-sigint-v1:ignored";
 pub const TOKEN_ERROR: &[u8] = b"pmacs-sigint-v1:error";
 
-/// Emitted on stderr by **every** stub, so a consumer's output can be
-/// searched for it: present ⇒ the child's stderr was surfaced.
+/// Emitted on stderr by the **branch-discriminating** cases — the
+/// cross-product rows and X1 — so a consumer's output can be searched
+/// for it: present ⇒ the child's stderr was surfaced.
+///
+/// **X3 and X4 deliberately carry their own payloads instead**, which
+/// is what makes them distinct inputs rather than duplicates of
+/// `1/E/empty` and `0/V/safe/bare`. An earlier revision gave every case
+/// this same sentinel and so shipped 45 entries over 43 distinct
+/// inputs; [`shared_cases`] now asserts uniqueness so that cannot
+/// recur silently.
 pub const SENTINEL: &str = "PMACS-CONFORMANCE-SENTINEL";
 
 /// The canonical wording the helper uses for `ignored`. X3 emits it
@@ -50,7 +58,8 @@ pub struct Case {
     pub expect: Outcome,
 }
 
-/// A `/bin/sh` stub reproducing one case, sentinel included.
+/// A `/bin/sh` stub reproducing one case exactly: its stdout bytes, its
+/// own stderr payload, and its status.
 #[must_use]
 pub fn stub_script(case: &Case) -> String {
     let octal = case.stdout.iter().fold(String::new(), |mut acc, b| {
@@ -185,5 +194,20 @@ pub fn shared_cases() -> Vec<Case> {
         stderr: "unrelated chatter on stderr".to_owned(),
         expect: Outcome::Safe,
     });
+    // The set must be 45 DISTINCT inputs, not merely 45 entries. A
+    // previous revision gave every case the same stderr, which silently
+    // collapsed X3 into `1/E/empty` and X4 into `0/V/safe/bare` — 45
+    // entries, 43 inputs, and two framing-specified cases quietly not
+    // exercised. Asserted here rather than in each suite so no consumer
+    // can forget it.
+    let mut seen = std::collections::HashSet::new();
+    for case in &out {
+        assert!(
+            seen.insert((case.status, case.stdout.clone(), case.stderr.clone())),
+            "duplicate conformance input at {}: (status, stdout, stderr) already present",
+            case.name
+        );
+    }
+    assert_eq!(seen.len(), out.len(), "every case must be a distinct input");
     out
 }
diff --git a/tests/gpu_invocation_acceptance.rs b/tests/gpu_invocation_acceptance.rs
index 7e86f44..314bfe5 100644
--- a/tests/gpu_invocation_acceptance.rs
+++ b/tests/gpu_invocation_acceptance.rs
@@ -289,9 +289,6 @@ mod crdt {
     /// surfaces it, a boundary failure must not.
     #[test]
     fn rd_precondition_validates_the_whole_conformance_set() {
-        // `self::` and NOT `crate::`: this file is ALSO compiled as a
-        // nested module of `gpu_initial_target_acceptance.rs`, where
-        // `crate::` is the outer test crate and has no `common`.
         // `super::` and NOT `crate::`: this file is ALSO compiled as a
         // nested module of `gpu_initial_target_acceptance.rs`, where
         // `crate::` is the outer test crate and has no `common`.