Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 806979)
+++ CHANGES.txt	(working copy)
@@ -719,6 +719,11 @@
     ValueSource, but takes care when composite (multi-segment) are
     passed to not double RAM usage in the FieldCache.  (Chris
     Hostetter, Mark Miller, Mike McCandless)
+
+37. LUCENE-1798: Added FieldCache.set/getInfoStream, which uses
+    FieldCacheSanityChecker to detect when a new cache entry has
+    caused additional insanity, printing the details at the time that
+    it happens.  (Chris Hostetter, Mike McCandless)
    
 Optimizations
 
Index: src/test/org/apache/lucene/search/TestFieldCache.java
===================================================================
--- src/test/org/apache/lucene/search/TestFieldCache.java	(revision 806979)
+++ src/test/org/apache/lucene/search/TestFieldCache.java	(working copy)
@@ -25,6 +25,8 @@
 import org.apache.lucene.util.LuceneTestCase;
 
 import java.io.IOException;
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
 
 public class TestFieldCache extends LuceneTestCase {
   protected IndexReader reader;
@@ -58,6 +60,18 @@
     reader = IndexReader.open(directory);
   }
 
+  public void testInfoStream() throws Exception {
+    try {
+      FieldCache cache = FieldCache.DEFAULT;
+      ByteArrayOutputStream bos = new ByteArrayOutputStream(1024);
+      cache.setInfoStream(new PrintStream(bos));
+      double [] doubles = cache.getDoubles(reader, "theDouble");
+      float [] floats = cache.getFloats(reader, "theDouble");
+      assertTrue(bos.toString().indexOf("WARNING") != -1);
+    } finally {
+      FieldCache.DEFAULT.purgeAllCaches();
+    }
+  }
 
   public void test() throws IOException {
     FieldCache cache = FieldCache.DEFAULT;
Index: src/java/org/apache/lucene/search/FieldCache.java
===================================================================
--- src/java/org/apache/lucene/search/FieldCache.java	(revision 806979)
+++ src/java/org/apache/lucene/search/FieldCache.java	(working copy)
@@ -25,6 +25,7 @@
 
 import java.io.IOException;
 import java.io.Serializable;
+import java.io.PrintStream;
 
 import java.text.DecimalFormat;
 
@@ -615,5 +616,13 @@
    */
   public abstract void purgeAllCaches();
 
+  /**
+   * If non-null, FieldCacheImpl will warn whenever
+   * entries are created that are not sane according to
+   * {@link FieldCacheSanityChecker}.
+   */
+  public void setInfoStream(PrintStream stream);
 
+  /** @see setInfoStream */
+  public PrintStream getInfoStream();
 }
Index: src/java/org/apache/lucene/search/FieldCacheImpl.java
===================================================================
--- src/java/org/apache/lucene/search/FieldCacheImpl.java	(revision 806979)
+++ src/java/org/apache/lucene/search/FieldCacheImpl.java	(working copy)
@@ -18,21 +18,22 @@
  */
 
 import java.io.IOException;
+import java.io.PrintStream;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
-import java.util.Set;
 import java.util.WeakHashMap;
 
-import org.apache.lucene.document.NumericField;
+import org.apache.lucene.document.NumericField; // javadoc
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.TermDocs;
 import org.apache.lucene.index.TermEnum;
 import org.apache.lucene.util.StringHelper;
+import org.apache.lucene.util.FieldCacheSanityChecker;
 
 /**
  * Expert: The default cache implementation, storing all values in memory.
@@ -208,12 +209,43 @@
             synchronized (readerCache) {
               innerCache.put(key, progress.value);
             }
+
+            // Only check if key.custom (the parser) is
+            // non-null; else, we check twice for a single
+            // call to FieldCache.getXXX
+            if (key.custom != null && wrapper != null) {
+              final PrintStream infoStream = wrapper.getInfoStream();
+              if (infoStream != null) {
+                printNewInsanity(infoStream, progress.value);
+              }
+            }
           }
           return progress.value;
         }
       }
       return value;
     }
+
+    private void printNewInsanity(PrintStream infoStream, Object value) {
+      final FieldCacheSanityChecker.Insanity[] insanities = FieldCacheSanityChecker.checkSanity(wrapper);
+      for(int i=0;i<insanities.length;i++) {
+        final FieldCacheSanityChecker.Insanity insanity = insanities[i];
+        final CacheEntry[] entries = insanity.getCacheEntries();
+        boolean found = false;
+        for(int j=0;j<entries.length;j++) {
+          if (entries[j].getValue() == value) {
+            found = true;
+            break;
+          }
+        }
+        if (found) {
+          // OK this insanity involves our entry
+          infoStream.println("WARNING: new FieldCache insanity created\nDetails: " + insanity.toString());
+          infoStream.println("\nStack:\n");
+          new Throwable().printStackTrace(infoStream);
+        }
+      }
+    }
   }
 
   /** Expert: Every composite-key in the internal cache is of this type. */
@@ -811,6 +843,15 @@
       return retArray;
     }
   };
-  
+
+  private volatile PrintStream infoStream;
+
+  public void setInfoStream(PrintStream stream) {
+    infoStream = stream;
+  }
+
+  public PrintStream getInfoStream() {
+    return infoStream;
+  }
 }