1. mess up (めちゃくちゃにする）
Meaning: to do something poorly
Examples
A – What’s wrong?
B – I messed up on the test!
_________
A – Tanaka san, I want you to do the report. Don’t mess it up!
B – I won’t!
2. get my wires crossed (勘違いする）
Meaning: To miscommunicate with someone
Examples
A – Why didn’t you come to the party?
B – The party? I thought it was tonight.
A – It was last night!
B – I must have got my wires crossed.
