Improve CheckInputs() comment about sig verification

src/main.cpp

@@ -1631,9 +1631,12 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsVi
         // Only if ALL inputs pass do we perform expensive ECDSA signature checks.
         // Helps prevent CPU exhaustion attacks.
 
-        // Skip ECDSA signature verification when connecting blocks
+        // Skip ECDSA signature verification when connecting blocks before the
-        // before the last block chain checkpoint. This is safe because block merkle hashes are
+        // last block chain checkpoint. Assuming the checkpoints are valid this
-        // still computed and checked, and any change will be caught at the next checkpoint.
+        // is safe because block merkle hashes are still computed and checked,
+        // and any change will be caught at the next checkpoint. Of course, if
+        // the checkpoint is for a chain that's invalid due to false scriptSigs
+        // this optimisation would allow an invalid chain to be accepted.
         if (fScriptChecks) {
             for (unsigned int i = 0; i < tx.vin.size(); i++) {
                 const COutPoint &prevout = tx.vin[i].prevout;