Sam and ROB, I agree with Sam that challenging or disagreeing with Chazal on divrei aggadah is kefirah. The Chazon Ish says so explicitly. I’m not sure if ROB was saying that Chazal were wrong ch’v, though, maybe he just meant that Chazal themselves would not have insisted on the shavuos being kept under certain circumstances.
Gavra, I think you’re right; I was referring to a theoretical circumstance where specific attention was being given to us, e.g. there were soldiers or policemen guarding a shul. But the default is probably different.