Index: generic/tclOO.c
==================================================================
--- generic/tclOO.c
+++ generic/tclOO.c
@@ -1278,10 +1278,11 @@
     }
     return;
 
   removeInstance:
     if (Deleted(clsPtr->thisPtr)) {
+	DelRef(clsPtr->instances.list[i]);
 	clsPtr->instances.list[i] = NULL;
     } else {
 	clsPtr->instances.num--;
 	if (i < clsPtr->instances.num) {
 	    clsPtr->instances.list[i] =

Index: tests/oo.test
==================================================================
--- tests/oo.test
+++ tests/oo.test
@@ -256,10 +256,23 @@
     B create C
 } -cleanup {
     rename test-oo-1.18 {}
     A destroy
 } -result ::C
+test oo-1.18.1 {Bug 75b8433707: memory leak in oo-1.18} -setup {
+    proc test-oo-1.18 {} return
+} -constraints memory -body {
+    leaktest {
+	oo::class create A
+	oo::class create B {superclass A}
+	oo::define B constructor {} {A create test-oo-1.18}
+	B create C
+	A destroy
+    }
+} -cleanup {
+    rename test-oo-1.18 {}
+} -result 0
 test oo-1.19 {basic test of OO functionality: teardown order} -body {
     oo::object create o
     namespace delete [info object namespace o]
     o destroy
     # Crashes on error