The Collaborative International Dictionary of English v.0.48:

Unreliable \Un`re*li"a*ble\, a.
   Not reliable; untrustworthy. See Reliable. --
   Un`re*li"a*ble*ness, n.
   [1913 Webster]

         Alcibiades . . . was too unsteady, and (according to
         Mr. Coleridge's coinage) "unreliable;" or perhaps, in
         more correct English, too "unrelyuponable." --De
                                                  Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unreliable
    adj 1: liable to be erroneous or misleading; "an undependable
           generalization" [syn: undependable, unreliable]
    2: not worthy of reliance or trust; "in the early 1950s
       computers were large and expensive and unreliable"; "an
       undependable assistant" [syn: unreliable, undependable]
       [ant: dependable, reliable]
    3: dangerously unstable and unpredictable; "treacherous winding
       roads"; "an unreliable trestle" [syn: treacherous,
       unreliable]
    4: lacking a sense of responsibility

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "unreliable":
   adrift, afloat, alternating, amorphous, capricious, changeable,
   changeful, coquettish, dangerous, desultory, deviable,
   disreputable, dizzy, doubtful, dubious, eccentric, erratic,
   faddish, faithless, false, falsehearted, fast and loose, fickle,
   fitful, flickering, flighty, flimsy, flirtatious, flitting,
   fluctuating, fly-by-night, freakish, garbled, giddy, hazardous,
   impetuous, impulsive, inaccurate, inauthentic, inconsistent,
   inconstant, indecisive, inexact, infirm, insecure, insubstantial,
   irregular, irresolute, irresponsible, light, mazy, mercurial,
   miscited, misquoted, misreported, misstated, moody, perfidious,
   perilous, precarious, provisional, questionable, quicksilver,
   rambling, restless, risky, rocky, roving, scatterbrained, shaky,
   shapeless, shifting, shifty, shuffling, skittish, slick, slippery,
   spasmodic, spineless, spurious, temporary, tentative, ticklish,
   tottery, toying, treacherous, tricky, trustless, unaccountable,
   unassured, unauthentic, unauthoritative, uncertain, uncontrolled,
   undependable, undisciplined, unfaithful, unfaithworthy, unfixed,
   unfounded, unhealthy, unpredictable, unrestrained, unsafe,
   unsettled, unsolid, unsound, unstable, unstable as water, unstaid,
   unsteadfast, unsteady, unsubstantial, unsure, untrue,
   untrustworthy, untrusty, vacillating, vagrant, variable, versatile,
   vicissitudinary, vicissitudinous, volatile, wandering, wanton,
   wavering, wavery, wavy, wayward, weak, whimsical, wishy-washy,
   workaday