diff --git a/src/cpp/ripple/TransactionCheck.cpp b/src/cpp/ripple/TransactionCheck.cpp
index 50b1a7f7c..433bd6960 100644
--- a/src/cpp/ripple/TransactionCheck.cpp
+++ b/src/cpp/ripple/TransactionCheck.cpp
@@ -7,5 +7,10 @@
 
 bool TransactionEngine::checkInvariants(TER result, const SerializedTransaction& txn, TransactionEngineParams params)
 {
+
+// 1) Make sure transaction changed account sequence number to correct value
+
+// 2) Make sure transaction didn't create XRP
+
 	return true;
 }
diff --git a/src/cpp/ripple/TransactionEngine.cpp b/src/cpp/ripple/TransactionEngine.cpp
index 1c0049fb2..1747f114d 100644
--- a/src/cpp/ripple/TransactionEngine.cpp
+++ b/src/cpp/ripple/TransactionEngine.cpp
@@ -176,12 +176,20 @@ TER TransactionEngine::applyTransaction(const SerializedTransaction& txn, Transa
 				if (isSetBit(params, tapOPEN_LEDGER))
 				{
 					if (!mLedger->addTransaction(txID, s))
+					{
+						cLog(lsFATAL) << "Tried to add transaction to open ledger that already had it";
 						assert(false);
+						throw std::runtime_error("Duplicate transaction applied");
+					}
 				}
 				else
 				{
 					if (!mLedger->addTransaction(txID, s, m))
-					assert(false);
+					{
+						cLog(lsFATAL) << "Tried to add transaction to ledger that already had it";
+						assert(false);
+						throw std::runtime_error("Duplicate transaction applied to closed ledger");
+					}
 
 					// Charge whatever fee they specified.
 					STAmount saPaid = txn.getTransactionFee();