* Ugh, nasty Heisenbug due to an uninitialiased variable. The bug

only caused a crash if the program was *not* invoked with a high
  verbosity level.

src/libstore/normalise.cc

@@ -1441,6 +1441,7 @@ Worker::Worker()
     /* Debugging: prevent recursive workers. */ 
     if (working) abort();
     working = true;
+    nrChildren = 0;
 }