diff --git a/src/core/slavebase.cpp b/src/core/slavebase.cpp
--- a/src/core/slavebase.cpp
+++ b/src/core/slavebase.cpp
@@ -213,6 +213,16 @@
         }
     }
 
+    void verfifyFinishedNotCalled(const char *cmdName)
+    {
+        KIO_STATE_ASSERT(m_state != FinishedCalled,
+                         Q_FUNC_INFO,
+                         qUtf8Printable(QStringLiteral("%1 called finished(), but it's not supposed to! Please fix the %2 KIO slave")
+                                        .arg(cmdName)
+                                        .arg(QCoreApplication::applicationName())));
+    }
+
+
     void verifyErrorFinishedNotCalled(const char *cmdName)
     {
         KIO_STATE_ASSERT(!finalState(),
@@ -1179,6 +1189,7 @@
         QIODevice::OpenMode mode = QFlag(i);
         d->m_state = d->InsideMethod;
         open(url, mode); //krazy:exclude=syscalls
+        d->verfifyFinishedNotCalled("open()");
         d->m_state = d->Idle;
     } break;
     case CMD_PUT: {
@@ -1362,22 +1373,26 @@
         KIO::filesize_t bytes;
         stream >> bytes;
         read(bytes);
+        d->verfifyFinishedNotCalled("read()");
         break;
     }
     case CMD_WRITE: {
         write(data);
+        d->verfifyFinishedNotCalled("write()");
         break;
     }
     case CMD_SEEK: {
         KIO::filesize_t offset;
         stream >> offset;
         seek(offset);
+        d->verfifyFinishedNotCalled("seek()");
         break;
     }
     case CMD_NONE:
         break;
     case CMD_CLOSE:
         close();                // must call finish(), which will set d->inOpenLoop=false
+        d->verifyState("close()");
         break;
     default:
         // Some command we don't understand.