diff --git a/src/validation.h b/src/validation.h
--- a/src/validation.h
+++ b/src/validation.h
@@ -330,7 +330,8 @@
  * Note that we guarantee that either the proof-of-work is valid on pblock, or
  * (and possibly also) BlockChecked will have been called.
  *
- * Call without cs_main held.
+ * May not be called with cs_main held. May not be called in a
+ * validationinterface callback.
  *
  * @param[in]   config  The global config.
  * @param[in]   pblock  The block we want to process.
@@ -347,7 +348,8 @@
 /**
  * Process incoming block headers.
  *
- * Call without cs_main held.
+ * May not be called with cs_main held. May not be called in a
+ * validationinterface callback.
  *
  * @param[in]  config        The config.
  * @param[in]  block         The block headers themselves.
@@ -422,6 +424,9 @@
 
 /**
  * Find the best known block, and make it the tip of the block chain
+ *
+ * May not be called with cs_main held. May not be called in a
+ * validationinterface callback.
  */
 bool ActivateBestChain(
     const Config &config, CValidationState &state,
@@ -632,7 +637,12 @@
 CBlockIndex *FindForkInGlobalIndex(const CChain &chain,
                                    const CBlockLocator &locator);
 
-/** Mark a block as precious and reorganize. */
+/**
+ * Mark a block as precious and reorganize.
+ *
+ * May not be called with cs_main held. May not be called in a
+ * validationinterface callback.
+ */
 bool PreciousBlock(const Config &config, CValidationState &state,
                    CBlockIndex *pindex);
 
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2907,6 +2907,9 @@
             // 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.
+            // Note that if a validationinterface callback ends up calling
+            // ActivateBestChain this may lead to a deadlock! We should
+            // probably have a DEBUG_LOCKORDER test for this in the future.
             SyncWithValidationInterfaceQueue();
         }