diff --git a/common/os.go b/common/os.go
index bab5fbce6..2d65683b8 100644
--- a/common/os.go
+++ b/common/os.go
@@ -2,6 +2,7 @@ package common
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/signal"
 )
@@ -26,3 +27,26 @@ func Exit(s string) {
 	fmt.Printf(s + "\n")
 	os.Exit(1)
 }
+
+// Writes to newBytes to filePath.
+// Guaranteed not to lose *both* oldBytes and newBytes,
+// (assuming that the OS is perfect)
+func AtomicWriteFile(filePath string, newBytes []byte) error {
+	// If a file already exists there, copy to filePath+".bak" (overwrite anything)
+	if _, err := os.Stat(filePath); !os.IsNotExist(err) {
+		fileBytes, err := ioutil.ReadFile(filePath)
+		if err != nil {
+			return fmt.Errorf("Failed to read file %v. %v", filePath, err)
+		}
+		err = ioutil.WriteFile(filePath+".bak", fileBytes, 0600)
+		if err != nil {
+			return fmt.Errorf("Failed to write file %v. %v", filePath+".bak", err)
+		}
+	}
+	// Write newBytes to filePath.
+	err := ioutil.WriteFile(filePath, newBytes, 0600)
+	if err != nil {
+		return fmt.Errorf("Failed to write file %v. %v", filePath, err)
+	}
+	return nil
+}
diff --git a/config/config.go b/config/config.go
index 23ed290b5..739b4d154 100644
--- a/config/config.go
+++ b/config/config.go
@@ -37,6 +37,7 @@ var defaultConfig = `# This is a TOML config file.
 
 Network = "tendermint_testnet0"
 ListenAddr = "0.0.0.0:8080"
+NodeName = "anon_dev"
 # First node to connect to.  Command-line overridable.
 SeedNode = "188.166.55.222:8080"
 
diff --git a/p2p/addrbook.go b/p2p/addrbook.go
index 893cf1c9e..f8abe28ee 100644
--- a/p2p/addrbook.go
+++ b/p2p/addrbook.go
@@ -319,14 +319,12 @@ func (a *AddrBook) saveToFile(filePath string) {
 		Addrs: addrs,
 	}
 
-	w, err := os.Create(filePath)
+	jsonBytes, err := json.MarshalIndent(aJSON, "", "\t")
 	if err != nil {
-		log.Error("Error opening file", "file", filePath, "error", err)
+		log.Error("Failed to save AddrBook to file", "err", err)
 		return
 	}
-	defer w.Close()
-	jsonBytes, err := json.MarshalIndent(aJSON, "", "\t")
-	_, err = w.Write(jsonBytes)
+	err = AtomicWriteFile(filePath, jsonBytes)
 	if err != nil {
 		log.Error("Failed to save AddrBook to file", "file", filePath, "error", err)
 	}
diff --git a/state/priv_validator.go b/state/priv_validator.go
index 17839a500..ed5dff713 100644
--- a/state/priv_validator.go
+++ b/state/priv_validator.go
@@ -99,8 +99,9 @@ func (privVal *PrivValidator) Save() {
 
 func (privVal *PrivValidator) save() {
 	jsonBytes := binary.JSONBytes(privVal)
-	err := ioutil.WriteFile(privVal.filename, jsonBytes, 0700)
+	err := AtomicWriteFile(privVal.filename, jsonBytes)
 	if err != nil {
+		// `@; BOOM!!!
 		panic(err)
 	}
 }