Index: SpanScorer.java
===================================================================
--- SpanScorer.java	(revision 234330)
+++ SpanScorer.java	(working copy)
@@ -49,11 +49,19 @@
     if (firstTime) {
       more = spans.next();
       firstTime = false;
+      doc = -1;
     }
 
     if (!more) return false;
 
     freq = 0.0f;
+
+    if (spans.doc() <= doc) {
+      throw new AssertionError("SpanScorer.next() A: spans " + spans
+                               + "\nadvanced from " + doc
+                               + " to " + spans.doc());
+    }
+
     doc = spans.doc();
 
     while (more && doc == spans.doc()) {
@@ -62,6 +70,12 @@
       more = spans.next();
     }
 
+    if (more && (spans.doc() <= doc)) {
+      throw new AssertionError("SpanScorer.next() B: spans " + spans
+                               + "\nadvanced from " + doc
+                               + " to " + spans.doc());
+    }
+
     return more || freq != 0.0f;
   }