diff --git a/pttodoer/src/main.rs b/pttodoer/src/main.rs
index 29b15e5..cf0210f 100644
--- a/pttodoer/src/main.rs
+++ b/pttodoer/src/main.rs
@@ -63,7 +63,7 @@ impl Flags {
    }
 }
 
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Clone)]
 pub struct DB {
    tasks_and_metadatas: Vec<TasksAndMetadata>,
    cwd: String,
@@ -243,6 +243,28 @@ impl DB {
 mod test_db {
    use super::*;
 
+   #[test]
+   fn conflicting_save_fails() {
+      let d = TempDir::new(&TS::now().to_string()).expect("failed to create a temp dir");
+      let d = d.path().display().to_string();
+
+      {
+         let p = format!("{}/f", &d);
+         eprintln!("p={}", p);
+         let mut f = std::fs::File::create(p).expect("failed to create a file in cwd");
+         f.write_all(b"- x").expect("failed to create a file");
+      }
+
+      let mut db = DB::new(d.clone()).expect("failed to open tempd");
+      assert_eq!(1, db.tasks_and_metadatas[0].tasks.len());
+      db.tasks_and_metadatas[0].tasks.0.push(Task::new());
+      let stale_db = db.clone();
+
+      db.save(false).expect("failed to save db with new task");
+      assert_eq!(false, stale_db.save(false).is_ok());
+   }
+
+
    #[test]
    fn edit_append_duplicate_keeps_both() {
       use std::io::Seek;