Well, my first thought was "FINALS!!!" but I can stay pretty calm through those some days, so I decided to pick something quirkier.
But there is something that would really scare me, and it would include the following in one room:
Me holding my violin
In a crowded room with these people.
And they are holding filled versions of these:
Yeah, I would be on pins and needles if this situation arose. Thankfully it doesn't happen too often, otherwise I would be a very cranky and high-strung person.