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]