It seems that two versions of Isaiah 8:11 exist.
One has "כחזקת היד" -- this is the version in the Aleppo (shown above) and Leningrad codices: http://www.studylight.org/desk/?sr=1&old_q=Isaiah+8%3A11&search_form_type=general&q1=Isaiah+8%3A11&s=0&t1=iw_ale&ns=0
One has "בחזקת היד " -- this is the version in Mikraot G'dolot and all the commentators there (Yonatan, Rashi, Radak, Metzudot).
Does anyone have any light to shed on this?