diff --git a/src/aacs/trace.rs b/src/aacs/trace.rs
index 4699d99..c499593 100644
--- a/src/aacs/trace.rs
+++ b/src/aacs/trace.rs
@@ -77,6 +77,11 @@ pub enum KeyNode {
     MatchedDisc,
     /// The source had no entry for this disc.
     NoEntry,
+    /// The source FAILED — it errored rather than reporting no entry. Distinct
+    /// from [`Self::NoEntry`]: an unreachable key server or an unreadable keydb is
+    /// a fixable condition, whereas "this disc is not in the database" is not, and
+    /// collapsing the two told the front-end the wrong cause.
+    SourceFailed,
     /// Pre-decrypted unit keys were found.
     FoundUnitKeys,
     /// A VUK was found.
diff --git a/src/keysource.rs b/src/keysource.rs
index a7c3c16..5854c3b 100644
--- a/src/keysource.rs
+++ b/src/keysource.rs
@@ -350,15 +350,25 @@ pub fn resolve_and_apply_traced(
                     outcome: KeyOutcome::NoKey,
                 });
             }
-            // Empty (no key here) or a source failure — both are "no key from
-            // this source"; move on to the next.
-            Ok(_) | Err(_) => {
+            // No key from this source either way, but WHY differs and the caller
+            // renders it: a source that errored is a fixable condition (server
+            // unreachable, keydb unreadable), while "no entry for this disc" is
+            // not. Recording both as NoEntry lost that distinction — the same one
+            // drive_unit_keys / drive_fmts_indexes were refactored to preserve.
+            Ok(_) => {
                 trace.keys.push(KeyStep {
                     who,
                     path: vec![KeyNode::NoEntry],
                     outcome: KeyOutcome::NoKey,
                 });
             }
+            Err(_) => {
+                trace.keys.push(KeyStep {
+                    who,
+                    path: vec![KeyNode::SourceFailed],
+                    outcome: KeyOutcome::NoKey,
+                });
+            }
         }
     }
     (false, trace)
diff --git a/src/mux/codec/dts.rs b/src/mux/codec/dts.rs
index 1d36714..79cc87f 100644
--- a/src/mux/codec/dts.rs
+++ b/src/mux/codec/dts.rs
@@ -690,12 +690,13 @@ fn dts_core_duration_ns(data: &[u8]) -> u64 {
 const DTS_PCMBLOCK_SAMPLES: u32 = 32;
 const DTS_SUBBAND_SAMPLES: u32 = 8;
 /// Number of LEGAL `AMODE` (channel-arrangement) codes. The 6-bit AMODE field
-/// (ETSI TS 102 114 §5.3.1) has 16 defined channel arrangements, codes 0-15;
-/// only 16-63 are reserved/user-defined and undecodable. ffmpeg's
-/// `ff_dca_channels[16] = {1,2,2,2,2,3,3,4,4,5,6,6,6,7,8,8}` confirms all 16 are
-/// decodable — codes 10-15 are the 6/7/8-channel layouts. A frame is dropped
-/// only when `audio_mode >= DTS_AMODE_COUNT` (i.e. a truly reserved 16-63 code);
-/// dropping a legal 10-15 multichannel core would silence recoverable audio.
+/// (ETSI TS 102 114 §5.3.1) defines 16 channel arrangements, codes 0-15, of
+/// which 10-15 are the 6/7/8-channel layouts; codes 16-63 are
+/// reserved/user-defined and undecodable. A frame is dropped only when
+/// `audio_mode >= DTS_AMODE_COUNT` (i.e. a truly reserved 16-63 code); dropping a
+/// legal 10-15 multichannel core would silence recoverable audio. The per-AMODE
+/// channel counts live in `DTS_AMODE_CH` in `mux/mp4/audio.rs`, cross-checked
+/// against the speaker masks in `DTS_AMODE_LAYOUT` by an invariant test.
 const DTS_AMODE_COUNT: u32 = 16;
 const DTS_LFE_FLAG_INVALID: u32 = 3;