The steamy love scene between Brian (Baldwin) and Jennifer (Jennifer Jason Leigh) on top of the Fire Engine in the latter portion of the movie:

The hose-bed of an average Fire Engine usually has two long, steel dividers in it to create three separate hose-beds to house three separate diameters of hose-line. The dividers stick up about a foot, and as our two lovebirds share intimacy atop the hose-bed, it is obvious that the dividers are not there; they enjoy a smooth and level bed of hose.

The bell hits and the Engine responds to the high-rise fire with the two on top. When they arrive on scene and a fire fighter starts pulling hose, he's surprised to see Jennifer's bra entangled on the line. He looks up and there's Brian smiling down at him, leaning with both hands on the miraculously appearing hose-bed dividers.

I've seen some bad slip-ups, but this one takes the cake.