B737-800 Flight Attendant reported a partial escape slide deployment resulting in maintenance being called and passengers deplaning.

Narrative

I was FA A. Upon seat belt sign being turned off; I got up from my jumpseat and turned lights to BRIGHT. Made FA verify and disarm doors PA. Did monitor and challenge. Lifted girt bar out of the floor brackets and clicked into top brackets in slide bustle. Stood up. Removed strap. Looked out window. Looked over to Forward service door first at the floor to make sure her girt bar was disarmed then to her strap which was moved to top then verbally said crosscheck. FA B looked at my door to verify the same and verbally said cross check. My girt bar was 100% in the top brackets. The gate agent (GA) knocked and I gave the thumbs up. I stepped back and when he started to open the door the girl bar unhooked from the right side and dropped. I yelled STOP. The girt bar wedged in the door frame and the slide bustle started to partially deploy. I said DON'T MOVE THAT DOOR. I told the CA and he called maintenance. Maintenance came up a ladder on AC Right - service door. We ended up deplaning thru AC Right Aft. Helped disabled pax and unaccompanied minors (UMs) deplane. I took pics. It clearly shows the girt bar was not on the floor; my door was disarmed correctly. Maintenance said they didn't understand what happened either because girt bar was definitely disarmed. I do not believe I could have done anything differently. I followed procedures. I disarmed and cross checked as trained.