A B737-900 Flight Attendant failed to properly disarm her emergency exit. An alert compatriot noted the error and the omission was promptly corrected.

Narrative

After parking; the Lead Flight Attendant stated to prepare for arrival and cross checked. I proceeded to get up and disarm the slide and remove the red strap across the window. I then turned on the POS to transmit and unlocked the lavatories. I left the galley and stood by the aft lavatories and watched the deplaning process. A few passengers came back to use the restroom. Deplaning was a slow process.When there were only a few passengers left another Flight Attendant suggested we check to make sure the slides were disarmed. I was not expecting anything to be wrong. She then turned and said that my slide was still armed but my red strap was undone. I was in total disbelief! I could not believe that when I bent down to disarm the slide that I did not. I have never removed the red strap prior to disarming the door. I have always been extremely careful in following procedures. I cannot explain why the slide was not disarmed. I can see myself bending down and disarming the door. There were no emergencies or passengers asking for things or anything to disrupt my routine. However; when I proceeded back to disarm the door the second time; I did notice that the hooks that the slide sits in when disarmed were loose. The bar was more or less just sitting in the hooks rather than needing to be pressed down. So now I wonder if I put the bar in the hooks but did not make certain they were in there securely and they slipped out. My error is that I did not double and triple check the disarmed door to make sure the bar stayed in the hooks and would not fall out. Another factor may have been that I was tired and ready to get off the airplane. What I learned in this situation was to just step back and review all of the procedures each and every time I am disarming/arming a door. Maybe it has become such a habit that I need to make sure I am absolutely aware of what I am doing at all times.