WordNet (r) 3.0 (2006):

lose sight of
    v 1: be no longer able to see; "We lost sight of the tower as
         pulled out of the harbor"

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "lose sight of":
   be caught napping, be neglectful, be negligent, default, disregard,
   draw a blank, fail, forget, gloss over, ignore, lapse, let go,
   let ride, let slide, let slip, lose, lose track of, neglect, nod,
   not care for, not get involved, not heed, not remember, not think,
   overlook, pass over, sleep, take for granted