diff --git a/main/src/main/scala/sbt/internal/Continuous.scala b/main/src/main/scala/sbt/internal/Continuous.scala
index 351490f80..be0c1cea1 100644
--- a/main/src/main/scala/sbt/internal/Continuous.scala
+++ b/main/src/main/scala/sbt/internal/Continuous.scala
@@ -648,30 +648,49 @@ private[sbt] object Continuous extends DeprecatedContinuous {
   private[this] class WatchExecutor(name: String) extends AutoCloseable {
     val id = new AtomicInteger(0)
     val threads = new java.util.Vector[Thread]
+    val closed = new AtomicBoolean(false)
     def submit[R](tag: String, f: => R): WatchExecutor.Future[R] = {
-      val queue = new LinkedBlockingQueue[Either[Unit, R]]
-      val runnable: Runnable = () => {
-        try Util.ignoreResult(queue.offer(Right(f)))
-        catch { case _: Exception => Util.ignoreResult(queue.offer(Left(()))) }
+      if (closed.get) new WatchExecutor.FutureFailed(new IllegalStateException("closed executor"))
+      else {
+        val queue = new LinkedBlockingQueue[Either[Unit, R]]
+        val runnable: Runnable = () => {
+          try Util.ignoreResult(queue.offer(if (closed.get) Left(()) else Right(f)))
+          catch { case _: Exception => Util.ignoreResult(queue.offer(Left(()))) }
+        }
+        val thread = new Thread(runnable, s"sbt-watch-$name-$tag")
+        thread.setDaemon(true)
+        thread.start()
+        threads.add(thread)
+        new WatchExecutor.FutureImpl(thread, queue, this)
       }
-      val thread = new Thread(runnable, s"sbt-watch-$name-$tag")
-      thread.setDaemon(true)
-      thread.start()
-      threads.add(thread)
-      new WatchExecutor.Future(thread, queue, this)
     }
     def removeThread(thread: Thread): Unit = Util.ignoreResult(threads.remove(thread))
-    def close(): Unit = {
-      threads.forEach(_.interrupt)
+    def close(): Unit = if (closed.compareAndSet(false, true)) {
+      threads.forEach { t =>
+        val deadline = 1.second.fromNow
+        while (t.isAlive && !deadline.isOverdue) {
+          t.interrupt()
+          t.join(10)
+        }
+        if (t.isAlive) System.err.println(s"Couldn't join watch thread $t")
+      }
       threads.clear()
     }
   }
   private object WatchExecutor {
-    final class Future[R](
+    sealed trait Future[R] extends Any {
+      def cancel(): Unit
+      def result: Try[R]
+    }
+    final class FutureFailed[R](t: Throwable) extends Future[R] {
+      def cancel(): Unit = {}
+      def result: Try[R] = Failure(t)
+    }
+    final class FutureImpl[R](
         thread: Thread,
         queue: LinkedBlockingQueue[Either[Unit, R]],
         executor: WatchExecutor
-    ) {
+    ) extends Future[R] {
       def cancel(): Unit = {
         thread.interrupt()
         executor.removeThread(thread)