Alright ... I put the will_resize check back in the code.  The `bitcask_pulse` test finds this problem with a single folder (the `fork_merge` step in the counterexample):

    [[{set,{var,11},{call,bitcask_pulse,bc_open,[true]}},
      {set,{var,13},
           {call,bitcask_pulse,puts,[{var,11},{1,16},<<0>>]}},
      {set,{var,18},{call,bitcask_pulse,bc_close,[{var,11}]}},
      {set,{var,23},{call,bitcask_pulse,incr_clock,[]}},
      {set,{var,24},{call,bitcask_pulse,bc_open,[true]}},
      {set,{var,26},
           {call,bitcask_pulse,puts,[{var,24},{4,13},<<0>>]}},
      {set,{var,31},{call,bitcask_pulse,fork_merge,[{var,24}]}},
      {set,{var,34},{call,bitcask_pulse,fold,[{var,24}]}},
      {set,{var,37},{call,bitcask_pulse,fold,[{var,24}]}}],
     {19747,13791,98974},
     [{events,[]}]]

The problem is that the fold in step # 37 sees the key 14 twice.