Wedrifid was saying that people aren't that likely to stampede or even evacuate if someone yells "fire" now. I think he's right. If I started shouting "fire" right now I think the people around me would think I'd snapped and would at the least ask what I was seeing that they weren't.

(If I actually wanted people to listen to me I would start by asking "do you smell fire?" to the person next to me. Two people shouting fire is much less likely to be someone hallucinating.)