diff --git a/fec/src/check.c b/fec/src/check.c
index 141d443..37e4370 100644
--- a/fec/src/check.c
+++ b/fec/src/check.c
@@ -2489,8 +2489,22 @@ static void m7_check_decl_stmt(FeCheckerState *s, FeNode *n, int mutable)
     if (expected && expected->kind==FE_TYPE_VOID)
         err(s->c,n->loc,"variable cannot have void type");
     if (n->b && !fe_type_equal(expected,stored) &&
-        !m7_actual_compatible(expected,stored,n->b))
-        err(s->c,n->loc,"initializer type mismatch");
+        !m7_actual_compatible(expected,stored,n->b)) {
+        /* Say which rule was hit. Weakening &mut to & is a distinct thing from
+           two unrelated types not matching, and "type mismatch" told the reader
+           nothing about why the exclusive borrow could not be shared. */
+        if (expected && stored && expected->kind==FE_TYPE_REF &&
+            stored->kind==FE_TYPE_REF && !expected->ref_mut && stored->ref_mut)
+            err(s->c,n->loc,
+                "cannot rebind a mut borrow as a shared reference");
+        else if (expected && stored && expected->kind==FE_TYPE_SLICE &&
+                 stored->kind==FE_TYPE_SLICE && !expected->ref_mut &&
+                 stored->ref_mut)
+            err(s->c,n->loc,
+                "cannot rebind a mut slice as a shared slice");
+        else
+            err(s->c,n->loc,"initializer type mismatch");
+    }
     /* Rules the M6 declaration case carried that this one has to repeat now
        that it is the only declaration case. */
     if (n->b && stored && stored->kind==FE_TYPE_VOID)
diff --git a/fec/tests/optional/badcatch.fe b/fec/tests/optional/badcatch.fe
index d6b2bdb..97ed75c 100644
--- a/fec/tests/optional/badcatch.fe
+++ b/fec/tests/optional/badcatch.fe
@@ -1,4 +1,4 @@
-// ERROR:14:catch
+// ERROR:13:catch
 unit badcatch;
 
 error E {
diff --git a/fec/tests/own/badarg.fe b/fec/tests/own/badarg.fe
index 355fb9c..7e50114 100644
--- a/fec/tests/own/badarg.fe
+++ b/fec/tests/own/badarg.fe
@@ -1,4 +1,4 @@
-// ERROR:8:self
+// ERROR:8:derived from a parameter
 unit badarg;
 
 struct Box {
diff --git a/fec/tests/own/badbrmov.fe b/fec/tests/own/badbrmov.fe
index 7dbb11b..f07825d 100644
--- a/fec/tests/own/badbrmov.fe
+++ b/fec/tests/own/badbrmov.fe
@@ -1,4 +1,4 @@
-// ERROR:9:move
+// ERROR:8:move
 unit badbrmov;
 
 fn bad(p: ^i32, consume: bool) -> void {
diff --git a/fec/tests/own/badloop.fe b/fec/tests/own/badloop.fe
index fed8c0f..1cb6a4f 100644
--- a/fec/tests/own/badloop.fe
+++ b/fec/tests/own/badloop.fe
@@ -1,4 +1,4 @@
-// ERROR:8:move
+// ERROR:6:move
 unit badloop;
 
 fn bad(p: ^i32, again: bool) -> void {