diff --git a/src/libstd/io/cursor.rs b/src/libstd/io/cursor.rs
index 60767ea478661..53347eb14db0d 100644
--- a/src/libstd/io/cursor.rs
+++ b/src/libstd/io/cursor.rs
@@ -89,6 +89,10 @@ pub struct Cursor<T> {
 impl<T> Cursor<T> {
     /// Creates a new cursor wrapping the provided underlying I/O object.
     ///
+    /// Cursor initial position is `0` even if underlying object (e.
+    /// g. `Vec`) is not empty. So writing to cursor starts with
+    /// overwriting `Vec` content, not with appending to it.
+    ///
     /// # Examples
     ///
     /// ```