diff --git a/CHANGELOG.md b/CHANGELOG.md
index f8e6e18367..a53b0c3cfc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 - @ for escaping keywords (e.g. `struct Foo { @type string }`). 
 - Windows Unicode fixes (V can now work with non-ASCII paths etc on Windows). 
 - Fix mutable args bugs + don't allow primitive arguments to be modified.  
+- Declaring a mutable variable and never modifying it results in a compilation error.
 
 
 ## V 0.1.16 
diff --git a/examples/tetris/tetris.v b/examples/tetris/tetris.v
index f0f0a5484c..7a6e634d44 100644
--- a/examples/tetris/tetris.v
+++ b/examples/tetris/tetris.v
@@ -226,7 +226,7 @@ fn (g mut Game) delete_completed_line(y int) {
 	for yy := y - 1; yy >= 1; yy-- {
 		for x := 1; x <= FieldWidth; x++ {
 			mut a := g.field[yy + 1]
-			mut b := g.field[yy]
+			b := g.field[yy]
 			a[x] = b[x]
 		}
 	}
diff --git a/vlib/builtin/array_test.v b/vlib/builtin/array_test.v
index 83c4b87415..efa9a6c77f 100644
--- a/vlib/builtin/array_test.v
+++ b/vlib/builtin/array_test.v
@@ -109,7 +109,7 @@ fn test_repeat() {
 		assert aa[9] == f32(1.1)
 	}
 	{
-		mut aa := [f64(1.1) ; 10]
+		aa := [f64(1.1) ; 10]
 		assert aa[0] == f64(1.1)
 		assert aa[5] == f64(1.1)
 		assert aa[9] == f64(1.1)
diff --git a/vlib/builtin/string_test.v b/vlib/builtin/string_test.v
index d0e6eb1155..6a91d693e9 100644
--- a/vlib/builtin/string_test.v
+++ b/vlib/builtin/string_test.v
@@ -141,7 +141,7 @@ fn test_clone() {
 	a += 'a'
 	a += 'a'
 	b := a
-	mut c := a.clone()
+	c := a.clone()
 	assert c == a
 	assert c == 'aaa'
 	assert b == 'aaa'
diff --git a/vlib/math/stats_test.v b/vlib/math/stats_test.v
index 09aab5c2d2..368930a54f 100644
--- a/vlib/math/stats_test.v
+++ b/vlib/math/stats_test.v
@@ -256,4 +256,5 @@ fn test_passing_empty() {
 	assert stats.min(data) == f64(0)
 	assert stats.max(data) == f64(0)
 	assert stats.range(data) == f64(0)
+}