Looking into the internals, it appears that the EXCHANGE locks the entire
    table for one exchange, at the same time as the other is trying to get
  one only on the partition. It's an exclusive, blocking lock, which requires
    the DBA person to kill the entire process if nothing happens. What I
  can't figure out is, why doesn't it do the exchange after it gets the lock?
    It just does nothing. We couldn't even execute a SELECT or DESCribe.
  One DBA believed it to be a bug with Oracle, and one that probably won't be
    fixed soon. It seems to me that any time this happens to a database,
  where no queries are possible, and no error is returned, it's safe to say
    it's something Oracle should look at. 

The command name is EXCHANGE so it seems reasonable it wants to manipulate 2 Tables. Both Tables I expect to be guarded with locks. EXCLUSIVE locks! If a process has one lock, it's of little utility. It needs both. Therefore after it has one lock it tries to get the other. Only to find the other lock hold by a process, that wants to EXCHANGE too. That process in turn needs another lock to complete the EXCHANGE. You can guess which process holds that lock.