At the end of the film Denzil Washington is given a new tie as a Christmas present. There is a knock at the door and his sister arrives, the next shot of Denzil Washington shows him wearing the tie. All of this occures in a matter of seconds.
Denzel didn't put the tie on, Sarah. He was paralyzed from the neck but could use his fingers and that's it. So someone like his Hispanic friend who gave it to him could have put it on. No slip-up.