YOU SAID:

I messed up with a member of the guard.

INTO JAPANESE

衛兵隊員とやらかしてしまった。

BACK INTO ENGLISH

I messed up with a guard.

INTO JAPANESE

警備員と間違えてしまいました。

BACK INTO ENGLISH

I mistook him for a security guard.

INTO JAPANESE

私は彼を警備員と間違えた。

BACK INTO ENGLISH

I mistook him for a security guard.