I was sitting on a plane in Dallas on my way to Guadalajara, Mexico earlier in the week. Everything was going well and the pilot announced we would be taking off shortly. I was sitting near the back of the plane. All of a sudden there was a loud whooshing sound that I've never heard before, and I've flown for many years. It was the emergency slide deploying. Apparently a workorder had been placed to have the lavatories cleaned. Unfortunately the maintenance guy showed up a bit late. The jet had already been buttoned up and the doors alarmed. He opened the door and deployed the slide. That quickly led to a number of people standing outside with radios and bewildered looks on their faces. Pilot came on and announced that we had a little problem. We weren't going anywhere until it was fixed. Fortunately they had another in a hanger on airport grounds and it was about an hour and a half of sitting on the plane before slide was removed and new package installed. Finally got into Guad about 1 AM. After that everything went fine, but after 25 or so years of flying that was a first. Maintenance guy wasn't hurt, but I suspect he may not be employed now.