When the swat team " moved in" on the Nakatomi Plaza building they spent precious moments trying to cut the lock, with a cutting torch on a glass door, and they all got shot from the inside by Hans Gruber's men. They were shot through the glass. I guess they didn't want to break the glass to get in.
Character stupidity is not a slip up. The fact that in the movie the local police were portrayed as kinda bumbling idiots further shows this wasn't a slip up...it was done on purpose to show the ineffectiveness of the local cops.
I don't think it was a slip-up either. They were trying to sneak into the building. They didn't know they were being seen on camera. Blasting through the glass would of surely give away there position.
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