OK, My turn to break things ...

The maximum vertex count for a mesh has been raised from 65000 to
2000000000L (yep, 2 billion!).

Please test!

Chris

source/blender/src/editmesh.c

@@ -6412,7 +6412,7 @@ void join_mesh(void)
 	/* that way the active object is always selected */ 
 	if(ok==0) return;
 	
-	if(totvert==0 || totvert>65000) return;
+	if(totvert==0 || totvert>2000000000L) return;
 	
 	if(okee("Join selected Meshes")==0) return;