diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -101,9 +101,10 @@
     fs::create_directories(pathTemp);
     gArgs.ForceSetArg("-datadir", pathTemp.string());
 
-    // Note that because we don't bother running a scheduler thread here,
-    // callbacks via CValidationInterface are unreliable, but that's OK,
-    // our unit tests aren't testing multiple parts of the code at once.
+    // We have to run a scheduler thread to prevent ActivateBestChain
+    // from blocking due to queue overrun.
+    threadGroup.create_thread(
+        boost::bind(&CScheduler::serviceQueue, &scheduler));
     GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
 
     g_mempool.setSanityCheck(1.0);
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -45,6 +45,7 @@
 #include "warnings.h"
 
 #include <atomic>
+#include <future>
 #include <sstream>
 #include <thread>
 
@@ -2856,6 +2857,17 @@
     CBlockIndex *pindexNewTip = nullptr;
     do {
         boost::this_thread::interruption_point();
+
+        if (GetMainSignals().CallbacksPending() > 10) {
+            // Block until the validation queue drains. This should largely
+            // never happen in normal operation, however may happen during
+            // reindex, causing memory blowup  if we run too far ahead.
+            std::promise<void> promise;
+            CallFunctionInValidationInterfaceQueue(
+                [&promise] { promise.set_value(); });
+            promise.get_future().wait();
+        }
+
         if (ShutdownRequested()) {
             break;
         }