diff --git a/abci/client/socket_client.go b/abci/client/socket_client.go
index 562676605..125005da9 100644
--- a/abci/client/socket_client.go
+++ b/abci/client/socket_client.go
@@ -26,16 +26,17 @@ var _ Client = (*socketClient)(nil)
 type socketClient struct {
 	cmn.BaseService
 
-	reqQueue    chan *ReqRes
-	flushTimer  *cmn.ThrottleTimer
+	addr        string
 	mustConnect bool
+	conn        net.Conn
+
+	reqQueue   chan *ReqRes
+	flushTimer *cmn.ThrottleTimer
 
 	mtx     sync.Mutex
-	addr    string
-	conn    net.Conn
 	err     error
-	reqSent *list.List
-	resCb   func(*types.Request, *types.Response) // listens to all callbacks
+	reqSent *list.List                            // list of requests sent, waiting for response
+	resCb   func(*types.Request, *types.Response) // called on all requests, if set.
 
 }
 
@@ -86,6 +87,7 @@ func (cli *socketClient) OnStop() {
 	cli.mtx.Lock()
 	defer cli.mtx.Unlock()
 	if cli.conn != nil {
+		// does this really need a mutex?
 		cli.conn.Close()
 	}
 
@@ -207,12 +209,15 @@ func (cli *socketClient) didRecvResponse(res *types.Response) error {
 	reqres.Done()            // Release waiters
 	cli.reqSent.Remove(next) // Pop first item from linked list
 
-	// Notify reqRes listener if set
+	// Notify reqRes listener if set (request specific callback).
+	// NOTE: it is possible this callback isn't set on the reqres object
+	// at this point, in which case it will be called after, when it is set.
+	// TODO: should we move this after the resCb call so the order is always consistent?
 	if cb := reqres.GetCallback(); cb != nil {
 		cb(res)
 	}
 
-	// Notify client listener if set
+	// Notify client listener if set (global callback).
 	if cli.resCb != nil {
 		cli.resCb(reqres.Request, res)
 	}
diff --git a/mempool/mempool.go b/mempool/mempool.go
index 3a7e3722f..73373a160 100644
--- a/mempool/mempool.go
+++ b/mempool/mempool.go
@@ -442,11 +442,13 @@ func (mem *Mempool) globalCb(req *abci.Request, res *abci.Response) {
 	mem.metrics.Size.Set(float64(mem.Size()))
 }
 
-// Request specific callback that should be set on individual reqRes objects to incorporate local information
-// when processing the response. This allows us to track the peer
-// that sent us this tx, so we can avoid sending it back to them.
-// NOTE: we could avoid the need for request-specific callbacks if we included the information
-// in the ABCI request!
+// Request specific callback that should be set on individual reqRes objects
+// to incorporate local information when processing the response.
+// This allows us to track the peer that sent us this tx, so we can avoid sending it back to them.
+// NOTE: alternatively, we could include this information in the ABCI request itself.
+//
+// External callers of CheckTx, like the RPC, can also pass an externalCb through here that is called
+// when all other response processing is complete.
 //
 // Used in CheckTxWithInfo to record PeerID who sent us the tx.
 func (mem *Mempool) reqResCb(tx []byte, peerID uint16, externalCb func(*abci.Response)) func(res *abci.Response) {
@@ -467,6 +469,8 @@ func (mem *Mempool) reqResCb(tx []byte, peerID uint16, externalCb func(*abci.Res
 	}
 }
 
+// Called from:
+//  - resCbFirstTime (lock not held) if tx is valid
 func (mem *Mempool) addTx(memTx *mempoolTx) {
 	e := mem.txs.PushBack(memTx)
 	mem.txsMap[sha256.Sum256(memTx.tx)] = e
@@ -474,6 +478,9 @@ func (mem *Mempool) addTx(memTx *mempoolTx) {
 	mem.metrics.TxSizeBytes.Observe(float64(len(memTx.tx)))
 }
 
+// Called from:
+//  - Update (lock held) if tx was committed
+// 	- resCbRecheck (lock not held) if tx was invalidated
 func (mem *Mempool) removeTx(tx types.Tx, elem *clist.CElement, removeFromCache bool) {
 	mem.txs.Remove(elem)
 	elem.DetachPrev()