field.cc:

  http://bugs.mysql.com/bug.php?id=2218
  updating utf-8 text field generate nonsense chars
  Fix for the above bug.

sql/field.cc

@@ -4567,17 +4567,19 @@ int Field_blob::store(const char *from,uint length,CHARSET_INFO *cs)
   }
   else
   {
+    bool was_conversion;
     char buff[80];
     String tmpstr(buff,sizeof(buff), &my_charset_bin);
+
     /* Convert character set if nesessary */
-    if (use_conversion(cs, field_charset))
+    if ((was_conversion= use_conversion(cs, field_charset)))
     { 
       tmpstr.copy(from, length, cs, field_charset);
       from= tmpstr.ptr();
       length=  tmpstr.length();
     }
     Field_blob::store_length(length);
-    if (table->copy_blobs || length <= MAX_FIELD_WIDTH)
+    if (was_conversion || table->copy_blobs || length <= MAX_FIELD_WIDTH)
     {						// Must make a copy
       if (from != value.ptr())			// For valgrind
       {