An emergency exit slide was deployed at the gate by a caterer when a Flight Attendant incorrectly understood the order to arm the doors had been given.

Narrative

The passengers had been boarded; and this was our last flight of a three day trip. After the Lead Flight Attendant made his announcements; I thought I heard him say 'prepare doors.' I did so and began walking up the aisle checking on passengers. At this time; I realized that the front entry door and cockpit doors remained open. I proceeded to walk forward to inquire. When I arrived to the forward galley; a caterer came from the rear of the aircraft and informed us that he had popped the slide. We inquired if he was injured; and he said no. I followed him to the aft galley as he told me that he had seen the orange strap; but that his sunglasses had been giving off a glare and he had opened the door anyway. The slide had been fully deployed and was facing upward at an angle as the door was only partially opened. The gate agent; the caterer; and I placed the slide inside the aft galley so the plane could be taken to another gate and a new plane brought to our current gate. We immediately contacted the Captain and began to secure another aircraft; deplane passengers; and continue our flight. The entire procedure was done effectively and efficiently and we departed less than 50 minutes late and all connections were good.I am eternally thankful that no one was injured due to my misunderstanding and know in the future I will be on high alert and double check myself. In this job; there are so many things going on at once in an attempt to board passengers; stow their bags; do our duties and depart on time. Sometimes; I get caught up in repetitive routines like simply shutting the doors and preparing for the demo that I have neglected a very important aspect. I should have immediately returned to the aft galley and 'unprepared' my doors as soon as I saw the forward entry door was open. As for the caterer; thankfully he was not injured; however; I would hope in the future he will never attempt to open a door with the strap across the window.