diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -37,6 +37,19 @@
 option(KIOCORE_ONLY "Only compile KIOCore, not KIOWidgets or anything that depends on it. This will disable support for cookies and passwordhandling (prompting and storing)." OFF)
 option(KIO_FORK_SLAVES "If set we start the slaves via QProcess. It's also possible to change this by setting the environment variable KDE_FORK_SLAVES." OFF)
 
+# Enable state assertion by default on Jenkins.
+# This option should eventually be dropped and always be enabled.
+set(ASSERT_SLAVE_STATES_DEFAULT OFF)
+if(DEFINED ENV{JENKINS_SERVER_COOKIE})
+    set(ASSERT_SLAVE_STATES_DEFAULT ON)
+endif()
+option(KIO_ASSERT_SLAVE_STATES
+    "Used to control whether slave state assertions are enabled. When not enabled only warnings are generated."
+    ${ASSERT_SLAVE_STATES_DEFAULT})
+if(KIO_ASSERT_SLAVE_STATES AND NOT CMAKE_BUILD_TYPE MATCHES "[Dd]ebug$")
+    message(FATAL_ERROR "KIO_ASSERT_SLAVE_STATES option enabled but not a Debug build. This makes no sense!")
+endif()
+
 option(BUILD_QCH "Build API documentation in QCH format (for e.g. Qt Assistant, Qt Creator & KDevelop)" OFF)
 add_feature_info(QCH ${BUILD_QCH} "API documentation in QCH format (for e.g. Qt Assistant, Qt Creator & KDevelop)")
 
diff --git a/src/core/config-kiocore.h.cmake b/src/core/config-kiocore.h.cmake
--- a/src/core/config-kiocore.h.cmake
+++ b/src/core/config-kiocore.h.cmake
@@ -8,3 +8,5 @@
 #define CMAKE_INSTALL_FULL_LIBEXECDIR_KF5 "${CMAKE_INSTALL_FULL_LIBEXECDIR_KF5}"
 
 #cmakedefine01 KIO_FORK_SLAVES
+
+#cmakedefine01 KIO_ASSERT_SLAVE_STATES
diff --git a/src/core/slavebase.cpp b/src/core/slavebase.cpp
--- a/src/core/slavebase.cpp
+++ b/src/core/slavebase.cpp
@@ -61,6 +61,12 @@
 #include <KAuth>
 #endif
 
+#if KIO_ASSERT_SLAVE_STATES
+#define KIO_STATE_ASSERT(cond, where, what) Q_ASSERT_X(cond, where, what)
+#else
+#define KIO_STATE_ASSERT(cond, where, what) qCWarning(KIO_CORE) << what
+#endif
+
 extern "C" {
     static void sigpipe_handler(int sig);
 }
@@ -123,6 +129,7 @@
     KIO::filesize_t totalSize;
     KRemoteEncoding *remotefile;
     enum { Idle, InsideMethod, FinishedCalled, ErrorCalled } m_state;
+    bool m_finalityCommand = true; // whether finished() or error() may/must be called
     QByteArray timeoutData;
 
     KPasswdServerClient *m_passwdServerClient;
@@ -185,18 +192,34 @@
         }
     }
 
+    bool finalState() const
+    {
+        return ((m_state == FinishedCalled) || (m_state == ErrorCalled));
+    }
+
     void verifyState(const char *cmdName)
     {
-        if ((m_state != FinishedCalled) && (m_state != ErrorCalled)) {
-            qCWarning(KIO_CORE) << cmdName << "did not call finished() or error()! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
+        KIO_STATE_ASSERT(finalState(),
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("%1 did not call finished() or error()! Please fix the %2 KIO slave")
+                                        .arg(cmdName)
+                                        .arg(QCoreApplication::applicationName())));
+        // Force the command into finished state. We'll not reach this for Debug builds
+        // that fail the assertion. For Release builds we'll have made sure that the
+        // command is actually finished after the verification regardless of what
+        // the slave did.
+        if (!finalState()) {
+            q->finished();
         }
     }
 
     void verifyErrorFinishedNotCalled(const char *cmdName)
     {
-        if (m_state == FinishedCalled || m_state == ErrorCalled) {
-            qCWarning(KIO_CORE) << cmdName << "called finished() or error(), but it's not supposed to! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
-        }
+        KIO_STATE_ASSERT(!finalState(),
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("%1 called finished() or error(), but it's not supposed to! Please fix the %2 KIO slave")
+                                        .arg(cmdName)
+                                        .arg(QCoreApplication::applicationName())));
     }
 
     KPasswdServerClient *passwdServerClient()
@@ -458,11 +481,22 @@
 
 void SlaveBase::error(int _errid, const QString &_text)
 {
+    KIO_STATE_ASSERT(d->m_finalityCommand,
+                     Q_FUNC_INFO,
+                     qUtf8Printable(QStringLiteral("error() was called, but it's not supposed to! Please fix the %1 KIO slave")
+                                    .arg(QCoreApplication::applicationName())));
+
     if (d->m_state == d->ErrorCalled) {
-        qCWarning(KIO_CORE) << "error() called twice! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
+        KIO_STATE_ASSERT(false,
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("error() called twice! Please fix the %1 KIO slave")
+                                        .arg(QCoreApplication::applicationName())));
         return;
     } else if (d->m_state == d->FinishedCalled) {
-        qCWarning(KIO_CORE) << "error() called after finished()! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
+        KIO_STATE_ASSERT(false,
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("error() called after finished()! Please fix the %1 KIO slave")
+                                        .arg(QCoreApplication::applicationName())));
         return;
     }
 
@@ -502,11 +536,22 @@
         d->pendingListEntries.clear();
     }
 
+    KIO_STATE_ASSERT(d->m_finalityCommand,
+                     Q_FUNC_INFO,
+                     qUtf8Printable(QStringLiteral("finished() was called, but it's not supposed to! Please fix the %2 KIO slave")
+                                    .arg(QCoreApplication::applicationName())));
+
     if (d->m_state == d->FinishedCalled) {
-        qCWarning(KIO_CORE) << "finished() called twice! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
+        KIO_STATE_ASSERT(false,
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("finished() called twice! Please fix the %1 KIO slave")
+                                        .arg(QCoreApplication::applicationName())));
         return;
     } else if (d->m_state == d->ErrorCalled) {
-        qCWarning(KIO_CORE) << "finished() called after error()! Please fix the" << QCoreApplication::applicationName() << "KIO slave";
+        KIO_STATE_ASSERT(false,
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("finished() called after error()! Please fix the %1 KIO slave")
+                                        .arg(QCoreApplication::applicationName())));
         return;
     }
 
@@ -1062,15 +1107,17 @@
     QUrl url;
     int i;
 
+    d->m_finalityCommand = true; // default
+
     switch (command) {
     case CMD_HOST: {
         QString passwd;
         QString host, user;
         quint16 port;
         stream >> host >> port >> user >> passwd;
         d->m_state = d->InsideMethod;
+        d->m_finalityCommand = false;
         setHost(host, port, user, passwd);
-        d->verifyErrorFinishedNotCalled("setHost()");
         d->m_state = d->Idle;
     } break;
     case CMD_CONNECT: {
@@ -1081,9 +1128,9 @@
     } break;
     case CMD_SLAVE_STATUS: {
         d->m_state = d->InsideMethod;
+        d->m_finalityCommand = false;
         slave_status();
         // TODO verify that the slave has called slaveStatus()?
-        d->verifyErrorFinishedNotCalled("slave_status()");
         d->m_state = d->Idle;
     } break;
     case CMD_SLAVE_CONNECT: {
@@ -1110,8 +1157,8 @@
     } break;
     case CMD_REPARSECONFIGURATION: {
         d->m_state = d->InsideMethod;
+        d->m_finalityCommand = false;
         reparseConfiguration();
-        d->verifyErrorFinishedNotCalled("reparseConfiguration()");
         d->m_state = d->Idle;
     } break;
     case CMD_CONFIG: {