Don't cross the line under any circumstances
This is a bug in The Elder Scrolls IV Oblivion in Sercen, I believe this has happen because their was a trap there which has been set off and it could be that the A.I. still registers it is active
Game shown is The Elder Scrolls IV: Oblivion
Check out Gamesplanet and pick up The Elder Scrolls IV: Oblivion GOTY Edition. Affiliate link down below