The Collaborative International Dictionary of English v.0.48:

Err \Err\ ([~e]r), v. i. [imp. & p. p. Erred; p. pr. & vb. n.
   Erring (?; 277, 85).] [F. errer, L. errare; akin to G.
   irren, OHG. irran, v. t., irr[=o]n, v. i., OS. irrien, Sw.
   irra, Dan. irre, Goth, a['i]rzjan to lead astray, airzise
   astray.]
   1. To wander; to roam; to stray. [Archaic] "Why wilt thou err
      from me?" --Keble.
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            What seemeth to you, if there were to a man an
            hundred sheep and one of them hath erred. --Wyclif
                                                  (Matt. xviii.
                                                  12).
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   2. To deviate from the true course; to miss the thing aimed
      at. "My jealous aim might err." --Shak.
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   3. To miss intellectual truth; to fall into error; to mistake
      in judgment or opinion; to be mistaken.
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            The man may err in his judgment of circumstances.
                                                  --Tillotson.
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   4. To deviate morally from the right way; to go astray, in a
      figurative sense; to do wrong; to sin.
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            Do they not err that devise evil?     --Prov. xiv.
                                                  22.
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   5. To offend, as by erring.
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WordNet (r) 3.0 (2006):

err
    v 1: to make a mistake or be incorrect [syn: err, mistake,
         slip]
    2: wander from a direct course or at random; "The child strayed
       from the path and her parents lost sight of her"; "don't
       drift from the set course" [syn: stray, err, drift]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "err":
   backslide, be in error, be mistaken, be wrong, blunder, bungle,
   commit sin, degenerate, deviate, divagate, do amiss, do wrong,
   drift, excurse, fall, fall into error, foozle, go adrift, go amiss,
   go astray, go awry, go wrong, goof, judge amiss, lapse, meander,
   misappreciate, misbelieve, miscalculate, miscompute, misconjecture,
   misconstrue, misdeem, misesteem, misestimate, misevaluate,
   misinterpret, misjudge, misreckon, miss, miss the mark, misthink,
   misvalue, offend, pererrate, ramble, rove, serve Mammon, sin, slip,
   slip up, snake, straggle, stray, stumble, transgress, trespass,
   trip, twist, twist and turn, wander, wind