I had a fenced off area with 1 gate with bots going in and out all the time, a recharge bot was pretty close to these bots, what happened was that the bot ran out of charge right as it got to the gate, opening it and making it impassable for anything else but not occupying it so it could be recharged, not actually sure if this was a bug but an interesting edge case anyway. simply adding a second gate fixed this.