mix up
mix up , noun
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.
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.
mix-up , noun
a mistake that results from taking one thing to be another.