diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1819720..235bbea 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -87,6 +87,7 @@ if(ENABLE_TEST)
     src/test/test1_ParseConfig.cpp
     src/test/test2_MatchKeySequence.cpp
     src/test/test3_Stage.cpp
+    src/test/test4_Fuzz.cpp
   )
 endif(ENABLE_TEST)
 
diff --git a/src/runtime/Stage.cpp b/src/runtime/Stage.cpp
index f863162..96c399d 100644
--- a/src/runtime/Stage.cpp
+++ b/src/runtime/Stage.cpp
@@ -98,6 +98,7 @@ KeySequence Stage::apply_input(const KeyEvent event) {
   for (auto& output : m_output_down)
     output.suppressed = false;
 
+  m_sequence_might_match = false;
   while (has_non_optional(m_sequence)) {
     // find first mapping which matches or might match sequence
     for (const auto& mapping : m_mappings) {
@@ -108,7 +109,6 @@ KeySequence Stage::apply_input(const KeyEvent event) {
         m_sequence_might_match = true;
         return std::move(m_output_buffer);
       }
-      m_sequence_might_match = false;
 
       if (result == MatchResult::match) {
         apply_output(get_output(mapping));
@@ -211,9 +211,7 @@ void Stage::update_output(const KeyEvent& event, KeyCode trigger) {
       it->suppressed = true;
     }
   }
-  else {
-    assert(event.state == KeyState::Down);
-
+  else if (event.state == KeyState::Down) {
     // reapply temporarily released
     auto reapplied = false;
     for (auto& output : m_output_down)
diff --git a/src/test/test4_Fuzz.cpp b/src/test/test4_Fuzz.cpp
new file mode 100644
index 0000000..1d3b782
--- /dev/null
+++ b/src/test/test4_Fuzz.cpp
@@ -0,0 +1,56 @@
+
+#include "test.h"
+#include "config/ParseConfig.h"
+#include "runtime/Stage.h"
+#include <set>
+#include <random>
+
+namespace {
+  Stage create_stage(const char* string) {
+    static auto parse_config = ParseConfig();
+    auto stream = std::stringstream(string);
+    auto config = parse_config(stream);
+    auto mappings = std::vector<Mapping>();
+    for (auto& command : config.commands)
+      mappings.push_back({
+        std::move(command.input),
+        std::move(command.default_mapping)
+      });
+    return Stage(std::move(mappings), { });
+  }
+} // namespace
+
+//--------------------------------------------------------------------
+
+TEST_CASE("Fuzz #1", "[Fuzz]") {
+  auto config = R"(
+    Ext = IntlBackslash
+    Ext         >>
+    Ext{W{K}}   >> 1
+    Ext{W{L}}   >> 2
+    Ext{W{J}}   >> 3
+    Ext{W{Any}} >>
+  )";
+  Stage stage = create_stage(config);
+
+  auto keys = std::vector<KeyCode>();
+  for (auto k : { "IntlBackslash", "W", "K", "L", "J", "I" })
+    keys.push_back(parse_input(k).front().key);
+  auto pressed = std::set<KeyCode>();
+
+  auto rand = std::mt19937(0);
+  auto dist = std::uniform_int_distribution<size_t>(0, keys.size() - 1);
+  for (auto i = 0; i < 1000; i++) {
+    const auto key = keys[dist(rand)];
+    if (auto it = pressed.find(key); it != end(pressed)) {
+      pressed.erase(it);
+      stage.apply_input({ key, KeyState::Up });
+    }
+    else {
+      pressed.insert(key);
+      stage.apply_input({ key, KeyState::Down });
+    }
+  }
+}
+
+//--------------------------------------------------------------------