These threads scare me, because I'm worried about what I would do in such a situation. I feel like so many people here are adament that they'd "never breath gas that smells off" and "they would be livid" and would "cause a sh*tshow" (these aren't actual quotes but basically paraphrases) and my response is...
I'm jealous of your resolve. I'm not sure I would do the same.
My first thought would be, "it's all in my head". If my buddy also said something seemed wrong, but the DM's kept going, I would probably say..."it's all in OUR heads." By the time other people mentioned it, I think I'd have the resolve of doing something about it, but before then I'm just not sure I would.
I guess it could be a sign of weak character. "I'm a sheep." Or maybe it's my general disposition to confrontation. Would I be willing to "non-confrontationally" walk myself off a cliff? Maybe it seems like that, I'm not sure.
So what's the goal of this post? I'm not exactly sure, but maybe it's that if you're in this situation and you ARE someone who doesn't take crap from anyone and are hardheaded, please stand up. Because there are other people who probably also agree with you but lack the fortitude to actually do anything about it. Be a shepherd for us sheep