I had a similar incident in 2003 at a Staybridge Lincolnshire (around Chicago): I checked in on the night and then got someone else's key and welcome voucher with someone's name on it. Now if I actually looked at the voucher carefully I would not noticed this error. However I was quite tired and all I did is grab them and went into the room, and I did not noticed anything as the room was unused and clean.
Then I went out and had late dinner, by the time I came back I saw police cars outside and then saw 2 policeman with a lady crying uncontrolledly. I went and asked what is going on. Then the policeman asked me if I checked in to the room xx_X which I said yes, and then they asked me for ID which I showed them. Then they asked how I got this room and I told them that I just checked in normally through reception. Then they asked me the credit card to use to book the room and I showed them my Green Corporate Amex.
What happened is that the lady was freaked out that the room is already checked in when she tried to check in on the same night, and the staff would not give her the key to her room that she already pre-paid for, and she suspect that it was some kind of fraud and her credit card being used etc etc etc and she cried and hence police was called in. Obviously after I told them what happened the hotel sorted out and reissued our keys and the police just left.
Obviously both her and myself complained to the management about this stuff up at the counter. But the management did not apologise at all. I told her to write a complaint to IHG/Priority Club about this, and I did this myself. I then got myself 20000 points from IHG as compensation!
Apart from the incident, the hotel itself is actually quite nice!
So whenever I stay at hotel, I will always lock the room, or at least put the secure chain on it! Also if there is some welcome voucher/envelope I will check the names on it!