Interesting how United could charged the bounced check back to your account. It was obvious that the check was not yours, but that of one of its employees. It's ridiculous that customers have to waster their time following up on things like this because some accounting departments to not take the time to do due diligence.
Hope you at least got a letter of apology from United. But, that would require extra work on someone's part and United is too big to really care.