I don't recall perfectly the details, but I was keeper playing scenario 3. But I noticed that there was a possibility for the game to end basically immediately. At some point the Investigators find a Crowbar to lift the manhole cover which also happens to be a melee weapon. One of the mythos cards I had as keep allowed me a chance to break the crowbar when it was used on my zombie. I didn't do this in the interest of keeping the game going. But if I had succeeded (its a skill check) then the game would immediately end for the investigators and they wouldn't be able to get to the next 3 clues.

I realize that the rules include no-win scenarios where they could just make it there job to make me lose (but they would have to wait a long time for the objective card). Is this an oversite, or is it intentional? To me it seems like maybe they let it goes cause it wicked unlikely (as you need to have drawn the right mythos card and they guy with the bar has to attack with it then fail the skill check of the card) but I don't think it should be possible at all.

TL;DR: You can break the crowbar that opens the manhole in scenario 3 with a mythos card in the right situation. This break the scenario, maybe?

Yes you can... but it only breaks if it is used as a weapon. An experienced player would know they probably will need the crowbar later and won't attack with it. If I were running a I'd never do that in the interest of the game. Or wait til after it is ued to open the door.