The Collaborative International Dictionary of English v.0.48:

mix up \mix up\ n.
   1. To confuse the identities of (two or more objects); to
      mistake (one object for another); as, at the family
      gathering he mixed up his two nieces, to their great
      amusement.
      [PJC]

   2. To mix together; -- usually implying a mistake, whether
      done intentionally or unintentionally; as, the mixed up
      this year's receipts with last year's, and it took hours
      to find the right ones.
      [PJC]

WordNet (r) 3.0 (2006):

mix up
    v 1: assemble without order or sense; "She jumbles the words
         when she is supposed to write a sentence" [syn: jumble,
         confuse, mix up]
    2: cause to be perplexed or confounded; "This problem stumped
       her" [syn: stump, mix up]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "mix up":
   addle, addle the wits, admix, alloy, amalgamate, anarchy, ball up,
   becloud, bedazzle, befuddle, bemingle, bewilder, blend, bollix up,
   botch, botchery, bother, bug, chaos, cloud, coalesce, combine,
   commingle, commix, commixture, complicate, compose, composite,
   compound, concoct, confound, confuse, confusion, conglomerate,
   daze, dazzle, derange, disarrange, discombobulate, discomfit,
   discompose, disconcert, disjoint, disorganize, disorient, disrupt,
   distemper, distract, disturb, dizzy, embarrass, embrangle,
   emulsify, entangle, fabulous formless darkness, flummox, flurry,
   fluster, flutter, fog, foul up, foul-up, fuddle, fumble, fuse,
   fusion, fuss, garble, hash, hassle, homogenize, immingle, immix,
   immixture, implicate, integrate, interblend, interfusion,
   interlace, interlard, intermingle, intermix, intermixture,
   intertwine, interweave, involve, jumble, knead, knot, license,
   louse up, maze, merge, mess up, mingle, mingle-mangle, misdeem,
   misidentify, misrule, mist, mix, moider, morass, muck up, muddle,
   mull, muss, perplex, perturb, pi, play hob with, pother, put out,
   raise hell, ramify, rattle, ravel, riffle, ruffle, scramble,
   screw up, screw-up, shambles, shuffle, snafu, snarl, snarl up,
   stir up, syncretize, tangle, throw into confusion, throw together,
   toss together, tumble, unsettle, upset, work