diff --git a/doc/api/repl.markdown b/doc/api/repl.markdown
index 487749250b04d9..b1a10f848a9948 100644
--- a/doc/api/repl.markdown
+++ b/doc/api/repl.markdown
@@ -219,6 +219,9 @@ The special variable `_` (underscore) contains the result of the last expression
     > _ += 1
     4
 
+*NOTE*: Explicitly assigning a value to `_` in the REPL can produce unexpected
+results.
+
 The REPL provides access to any variables in the global scope. You can expose
 a variable to the REPL explicitly by assigning it to the `context` object
 associated with each `REPLServer`.  For example: