All red sewer exitgirds bugged by the fact that u cant enter them while u had 1 sec combat timer on before (im not even sure if there is a combat timer in the sewers , i tried to catch the 1 sec one but i wasnt lucky) , you can walk as much as you want on them after you had a 1 sec combat timer i guess , doesnt matter which grid ... only thing which solves this is going off from the grid and reenter it. that bug is in there like forever also.
(http://i.imgur.com/zLUrDuW.gif)