diff --git a/modules/core/src/main/scala/tut/Tut.scala b/modules/core/src/main/scala/tut/Tut.scala
index 079fd3d..dd75b98 100644
--- a/modules/core/src/main/scala/tut/Tut.scala
+++ b/modules/core/src/main/scala/tut/Tut.scala
@@ -14,12 +14,16 @@ object Tut {
 
   def file(in: File): Tut[Unit] = for {
     lines <- FileIO.lines(in).liftIO[Tut]
-    _     <- lines.zipWithIndex.traverse { case (l, num) => line(l, num + 1) }
+    _     <- lines.zipWithIndexAndNext(false)(nextCloses)
+                  .traverse { case (l, nextCloses, num) => line(l, num + 1, nextCloses) }
   } yield ()
 
   // Private, utility methods
 
-  private def line(text: String, lineNumber: Int): Tut[Unit] =
+  private val nextCloses: String => Boolean =
+    _.trim.startsWith("```")
+
+  private def line(text: String, lineNumber: Int, nextCloses: Boolean): Tut[Unit] =
     for {
       s <- Tut.state
       inv = s.mods.filter(m => m == Invisible || m == Passthrough || m.isInstanceOf[Decorate])
@@ -35,11 +39,11 @@ object Tut {
           s
         }
       }.liftIO[Tut]
-      _ <- s.isCode.fold(interp(text, lineNumber), out(fixShed(text, mods ++ inv)))
+      _ <- s.isCode.fold(interp(text, lineNumber, nextCloses), out(fixShed(text, mods ++ inv)))
       _ <- checkBoundary(text, "```tut", true, mods)
     } yield ()
 
-  private def interp(text: String, lineNum: Int): Tut[Unit] =
+  private def interp(text: String, lineNum: Int, nextCloses: Boolean): Tut[Unit] =
     Tut.state >>= { s =>
       (text.trim.nonEmpty || s.partial.nonEmpty || s.mods(Silent)).whenM[Tut,Unit] {
         for {
@@ -50,6 +54,7 @@ object Tut {
           r <- // in 2.13 it's an error to interpret an empty line of text, evidently
                if (text.trim.isEmpty && s.partial.isEmpty) success
                else IO(s.imain.interpret(s.partial + "\n" + text)).liftIO[Tut] >>= {
+                 case Results.Incomplete if nextCloses => error(lineNum, Some("incomplete input in code block, missing brace or paren?"))
                  case Results.Incomplete => incomplete(text)
                  case Results.Success    => if (s.mods(Fail)) error(lineNum, Some("failure was asserted but no failure occurred")) else success
                  case Results.Error      => if (s.mods(NoFail) || s.mods(Fail)) success else error(lineNum)
diff --git a/modules/core/src/main/scala/tut/felix/Syntax.scala b/modules/core/src/main/scala/tut/felix/Syntax.scala
index b24d8b7..f9b5235 100644
--- a/modules/core/src/main/scala/tut/felix/Syntax.scala
+++ b/modules/core/src/main/scala/tut/felix/Syntax.scala
@@ -2,6 +2,8 @@ package tut.felix
 
 import java.io.OutputStream
 
+import scala.annotation.tailrec
+
 trait Syntax {
 
   implicit class MonadOps[M[_], A](ma: M[A])(implicit M: Monad[M]) {
@@ -35,6 +37,18 @@ trait Syntax {
   implicit class ListOps[A](as: List[A]) {
     def traverse[M[_]: Monad, B](f: A => M[B]): M[List[B]] =
       as.foldRight(List.empty[B].point[M])((a, mlb) => f(a).flatMap(b => mlb.map(b :: _)))
+
+    def zipWithIndexAndNext[B](z: => B)(f: A => B): List[(A, B, Int)] = {
+      @tailrec
+      def fold(xs: List[(A, Int)], acc: Vector[(A, B, Int)]): Vector[(A, B, Int)] =
+        xs match {
+          case (x, i) :: (xs @ ((y, _) :: _)) => fold(xs, acc :+ ((x, f(y), i)))
+          case (x, i) :: Nil => acc :+ ((x, z, i))
+          case Nil => Vector.empty
+        }
+
+      fold(as.zipWithIndex, Vector.empty).toList
+    }
   }
 
   implicit class BooleanOps(b: Boolean) {
diff --git a/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/build.sbt b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/build.sbt
new file mode 100644
index 0000000..784c014
--- /dev/null
+++ b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/build.sbt
@@ -0,0 +1,5 @@
+enablePlugins(TutPlugin)
+
+scalaVersion := sys.props("scala.version")
+
+scalacOptions += "-language:higherKinds"
diff --git a/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/project/plugins.sbt b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/project/plugins.sbt
new file mode 100644
index 0000000..bbe56cd
--- /dev/null
+++ b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/project/plugins.sbt
@@ -0,0 +1 @@
+addSbtPlugin("org.tpolecat" % "tut-plugin" % sys.props("project.version"))
diff --git a/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/src/main/tut/test.md b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/src/main/tut/test.md
new file mode 100644
index 0000000..61353ae
--- /dev/null
+++ b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/src/main/tut/test.md
@@ -0,0 +1,23 @@
+Ok
+
+```tut
+1 + 2
+```
+
+Open paren
+
+```tut
+println(
+```
+
+Open brace
+
+```tut
+1 match {
+```
+
+Incomplete class def
+
+```tut
+class Foo {
+```
diff --git a/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/test b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/test
new file mode 100644
index 0000000..45d43cf
--- /dev/null
+++ b/modules/tests/src/sbt-test/tut/test-14-unclosed-expr/test
@@ -0,0 +1 @@
+-> tut