Index: /trunk/src/VBox/Main/src-server/ApplianceImplIO.cpp
===================================================================
--- /trunk/src/VBox/Main/src-server/ApplianceImplIO.cpp	(revision 42388)
+++ /trunk/src/VBox/Main/src-server/ApplianceImplIO.cpp	(revision 42389)
@@ -1081,4 +1081,6 @@
 //        RTPrintf("Gap Read uOffset: %7lu cbRead: %7lu = %7lu\n", uOffset, cbRead, uOffset + cbRead);
     }
+    else if (uOffset < pInt->cbCurAll)
+        AssertMsgFailed(("Jumping backwards is not possible, sequential access is supported only\n"));
 
     size_t cbAllRead = 0;