diff --git a/abci/client/local_client.go b/abci/client/local_client.go
index 27883ee1f..c30427e37 100644
--- a/abci/client/local_client.go
+++ b/abci/client/local_client.go
@@ -15,6 +15,8 @@ var _ Client = (*localClient)(nil)
 type localClient struct {
 	service.BaseService
 
+	// TODO: remove `mtx` to increase concurrency.
+	// CONTRACT: The application should protect itself from concurrency as an abci server.
 	mtx *tmsync.Mutex
 	types.Application
 	Callback
@@ -92,9 +94,6 @@ func (app *localClient) DeliverTxAsync(params types.RequestDeliverTx) *ReqRes {
 }
 
 func (app *localClient) CheckTxAsync(req types.RequestCheckTx) *ReqRes {
-	// CONTRACT: Application should handle concurrent `CheckTx`
-	// In this abci client layer, we don't protect `CheckTx` with a mutex for concurrency
-
 	res := app.Application.CheckTx(req)
 	return app.callback(
 		types.ToRequestCheckTx(req),
@@ -103,9 +102,6 @@ func (app *localClient) CheckTxAsync(req types.RequestCheckTx) *ReqRes {
 }
 
 func (app *localClient) BeginRecheckTxAsync(req types.RequestBeginRecheckTx) *ReqRes {
-	app.mtx.Lock()
-	defer app.mtx.Unlock()
-
 	res := app.Application.BeginRecheckTx(req)
 	return app.callback(
 		types.ToRequestBeginRecheckTx(req),
@@ -114,9 +110,6 @@ func (app *localClient) BeginRecheckTxAsync(req types.RequestBeginRecheckTx) *Re
 }
 
 func (app *localClient) EndRecheckTxAsync(req types.RequestEndRecheckTx) *ReqRes {
-	app.mtx.Lock()
-	defer app.mtx.Unlock()
-
 	res := app.Application.EndRecheckTx(req)
 	return app.callback(
 		types.ToRequestEndRecheckTx(req),
@@ -258,9 +251,6 @@ func (app *localClient) DeliverTxSync(req types.RequestDeliverTx) (*types.Respon
 }
 
 func (app *localClient) CheckTxSync(req types.RequestCheckTx) (*types.ResponseCheckTx, error) {
-	// CONTRACT: Application should handle concurrent `CheckTx`
-	// In this abci client layer, we don't protect `CheckTx` with a mutex for concurrency
-
 	res := app.Application.CheckTx(req)
 	return &res, nil
 }
diff --git a/mempool/clist_mempool.go b/mempool/clist_mempool.go
index bd85ff675..5d6b49c70 100644
--- a/mempool/clist_mempool.go
+++ b/mempool/clist_mempool.go
@@ -20,7 +20,6 @@ import (
 	"github.com/line/ostracon/p2p"
 	"github.com/line/ostracon/proxy"
 	"github.com/line/ostracon/types"
-	"github.com/pkg/errors"
 )
 
 // TxKeySize is the size of the transaction key index
@@ -591,7 +590,7 @@ func (mem *CListMempool) Update(
 	block *types.Block,
 	deliverTxResponses []*abci.ResponseDeliverTx,
 	preCheck PreCheckFunc,
-) error {
+) (err error) {
 	// Set height
 	mem.height = block.Height
 	mem.notifiedTxsAvailable = false
@@ -624,54 +623,47 @@ func (mem *CListMempool) Update(
 		}
 	}
 
-	// Either recheck non-committed txs to see if they became invalid
-	// or just notify there're some txs left.
-	recheckStartTime := time.Now().UnixNano()
-	if mem.Size() > 0 {
-		if mem.config.Recheck {
-			mem.logger.Info("recheck txs", "numtxs", mem.Size(), "height", block.Height)
-			res, err := mem.proxyAppConn.BeginRecheckTxSync(abci.RequestBeginRecheckTx{
-				Header: types.OC2PB.Header(&block.Header),
-			})
-			if res.Code == abci.CodeTypeOK && err == nil {
-				mem.recheckTxs()
-				res2, err2 := mem.proxyAppConn.EndRecheckTxSync(abci.RequestEndRecheckTx{Height: block.Height})
-				if res2.Code != abci.CodeTypeOK {
-					return errors.New("the function EndRecheckTxSync does not respond CodeTypeOK")
-				}
-				if err2 != nil {
-					return errors.Wrap(err2, "the function EndRecheckTxSync returns an error")
-				}
-			} else {
-				if res.Code != abci.CodeTypeOK {
-					return errors.New("the function BeginRecheckTxSync does not respond CodeTypeOK")
-				}
-				if err != nil {
-					return errors.Wrap(err, "the function BeginRecheckTxSync returns an error")
-				}
-			}
+	if mem.config.Recheck {
+		// recheck non-committed txs to see if they became invalid
+		recheckStartTime := time.Now().UnixNano()
 
-			// At this point, mem.txs are being rechecked.
-			// mem.recheckCursor re-scans mem.txs and possibly removes some txs.
-			// Before mem.Reap(), we should wait for mem.recheckCursor to be nil.
-		} else {
-			mem.notifyTxsAvailable()
+		_, err = mem.proxyAppConn.BeginRecheckTxSync(abci.RequestBeginRecheckTx{
+			Header: types.OC2PB.Header(&block.Header),
+		})
+		if err != nil {
+			mem.logger.Error("error in proxyAppConn.BeginRecheckTxSync", "err", err)
 		}
-	}
-	recheckEndTime := time.Now().UnixNano()
 
-	recheckTimeMs := float64(recheckEndTime-recheckStartTime) / 1000000
-	mem.metrics.RecheckTime.Set(recheckTimeMs)
+		mem.logger.Info("recheck txs", "numtxs", mem.Size(), "height", block.Height)
+		mem.recheckTxs()
+
+		_, err = mem.proxyAppConn.EndRecheckTxSync(abci.RequestEndRecheckTx{Height: block.Height})
+		if err != nil {
+			mem.logger.Error("error in proxyAppConn.EndRecheckTxSync", "err", err)
+		}
+
+		recheckEndTime := time.Now().UnixNano()
+
+		recheckTimeMs := float64(recheckEndTime-recheckStartTime) / 1000000
+		mem.metrics.RecheckTime.Set(recheckTimeMs)
+
+		// At this point, mem.txs are being rechecked.
+		// mem.recheckCursor re-scans mem.txs and possibly removes some txs.
+		// Before mem.Reap(), we should wait for mem.recheckCursor to be nil.
+	} else if mem.Size() > 0 {
+		// just notify there're some txs left.
+		mem.notifyTxsAvailable()
+	}
 
 	// Update metrics
 	mem.metrics.Size.Set(float64(mem.Size()))
 
-	return nil
+	return err
 }
 
 func (mem *CListMempool) recheckTxs() {
 	if mem.Size() == 0 {
-		panic("recheckTxs is called, but the mempool is empty")
+		return
 	}
 
 	mem.recheckCursor = mem.txs.Front()