EMB 170 First Officer reported that an aft door arming lever was stuck and escape slide deployed slightly. Maintenance was called.

Narrative

The crew arrived to the plane and began preflighting. I did the walk around and saw there were no irregularities or problems. There were 2 small writeups concerning seat trim and radio static; both of which were cleared and fixed by Maintenance (MX). We continued with our preflight duties; loaded the passengers and bags; and received aircraft information. The captain gave the FA's the clear to close and we began the Before Start checklist. During this; the FAs called on the intercom and said the R2 door arming lever was stuck and would not go down. Both FAs were next to the affected door at this point. The captain suggested over the intercom it may be an alignment issue; and to ensure the door was unarmed; then to try and open and close the door again. The FAs tried this and realized the slide had deployed slightly and did not touch the door again and considered it a live door from that point. After learning this; we called maintenance; deplaned; and were reassigned to a new tail. The FA A and myself both agreed that there were no irregularities during our preflight duties; both on the interior and exterior of the door. One of the MX personnel said that it was likely that the slide was deployed to the degree it was before the FA went to arm it. In hindsight; we should have called MX as soon as the FAs had issues arming the door.