storage/ndb/src/kernel/blocks/pgman.cpp:
  a) in one case pl_queue.remove(ptr) was not followed
  by state &= ~ Page_entry::ONQUEUE.
  
  b) when collecting initial hot entries have to remove
  from queue if somehow got there.
  
  b) is easy to get with large buffer cache (256M).
  a) or b) is probably cause of bug#20612.