Index: src/vdbe.h
==================================================================
--- src/vdbe.h
+++ src/vdbe.h
@@ -259,10 +259,11 @@
 void sqlite3VdbeSetVarmask(Vdbe*, int);
 #ifndef SQLITE_OMIT_TRACE
   char *sqlite3VdbeExpandSql(Vdbe*, const char*);
 #endif
 int sqlite3MemCompare(const Mem*, const Mem*, const CollSeq*);
+int sqlite3BlobCompare(const Mem*, const Mem*);
 
 void sqlite3VdbeRecordUnpack(KeyInfo*,int,const void*,UnpackedRecord*);
 int sqlite3VdbeRecordCompare(int,const void*,UnpackedRecord*);
 int sqlite3VdbeRecordCompareWithSkip(int, const void *, UnpackedRecord *, int);
 UnpackedRecord *sqlite3VdbeAllocUnpackedRecord(KeyInfo*);

Index: src/vdbeaux.c
==================================================================
--- src/vdbeaux.c
+++ src/vdbeaux.c
@@ -3910,11 +3910,11 @@
 /*
 ** Compare two blobs.  Return negative, zero, or positive if the first
 ** is less than, equal to, or greater than the second, respectively.
 ** If one blob is a prefix of the other, then the shorter is the lessor.
 */
-static SQLITE_NOINLINE int sqlite3BlobCompare(const Mem *pB1, const Mem *pB2){
+SQLITE_NOINLINE int sqlite3BlobCompare(const Mem *pB1, const Mem *pB2){
   int c;
   int n1 = pB1->n;
   int n2 = pB2->n;
 
   /* It is possible to have a Blob value that has some non-zero content

Index: src/vdbemem.c
==================================================================
--- src/vdbemem.c
+++ src/vdbemem.c
@@ -884,11 +884,25 @@
 void sqlite3VdbeMemAboutToChange(Vdbe *pVdbe, Mem *pMem){
   int i;
   Mem *pX;
   for(i=0, pX=pVdbe->aMem; i<pVdbe->nMem; i++, pX++){
     if( pX->pScopyFrom==pMem ){
-      pX->flags |= MEM_Undefined;
+      /* If pX is marked as a shallow copy of pMem, then verify that
+      ** no significant changes have been made to pX since the OP_SCopy.
+      ** A significant change would indicated a missed call to this
+      ** function for pX.  Minor changes, such as adding or removing a
+      ** dual type, are allowed, as long as the underlying value is the
+      ** same. */
+      u16 mFlags = pMem->flags & pX->flags;
+      assert( (mFlags&MEM_Int)==0 || pMem->u.i==pX->u.i );
+      assert( (mFlags&MEM_Real)==0 || pMem->u.r==pX->u.r );
+      assert( (mFlags&MEM_Str)==0  || (pMem->n==pX->n && pMem->z==pX->z) );
+      assert( (mFlags&MEM_Blob)==0  || sqlite3BlobCompare(pMem,pX)==0 );
+      
+      /* pMem is the register that is changing.  But also mark pX as
+      ** undefined so that we can quickly detect the shallow-copy error */
+      pX->flags = MEM_Undefined;
       pX->pScopyFrom = 0;
     }
   }
   pMem->pScopyFrom = 0;
 }