Index: DisjunctionSumScorer.java
===================================================================
--- DisjunctionSumScorer.java	(revision 225556)
+++ DisjunctionSumScorer.java	(working copy)
@@ -149,6 +149,13 @@
       nrMatchers = 1;
       do { // Until all subscorers are after currentDoc
         if (top.next()) {
+          if (top.doc() <= currentDoc) {
+            throw new AssertionError("Scorer: " + top
+                                     + "\n advanced from doc " + currentDoc
+                                     + " to doc " + top.doc()
+                                     + " after next() returned true."
+                                     );
+          }
           scorerQueue.adjustTop();
         } else {
           scorerQueue.pop();
@@ -167,6 +174,16 @@
         } else {
           currentScore += top.score();
           nrMatchers++;
+          if (nrMatchers > nrScorers) {
+            throw new AssertionError("nrMatchers: " + nrMatchers
+                                     + " bigger than nrScorers: " + nrScorers
+                                     + "\nminimumNrMatchers: " + minimumNrMatchers
+                                     + ", currentDoc: " + currentDoc
+                                     + ", currentScore: " + currentScore
+                                     + "\ntop: " + top
+                                     + "\nscorerQueue.size(): " + scorerQueue.size()
+                                     );
+          }
         }
       } while (true);