Issue #2511 has been updated by Usaku NAKAMURA.

Status changed from Open to Assigned
Assigned to set to Kouji Takao

It seems the bug of libreadline.
FYI, ruby 1.8.8dev has that the same problem. 
----------------------------------------
http://redmine.ruby-lang.org/issues/show/2511

----------------------------------------
http://redmine.ruby-lang.org