Unfortunately the actor owed many evil people large sums of money, not to mention pissed off several unsavory organizations, so he and the players have been hounded by assassins at every step. They eventually made it to the theatre and were trying to come up with a plan to protect the actor through the performance when I mentioned - repeatedly - that there were a number of veiled and masked priestesses performing rituals to cleanse the theatre of evil spirits. I also mentioned - repeatedly - that the priestesses faces were hidden and that they seemed a little suspicious. Two of the player characters even found and killed another assassin, and discovered one of the priestess' masks on the body.
Of course, they did absolutely nothing to investigate the potential threat from the masked vestal virgins. It should come as no surprise to anyone reading that two of the priestesses eventually attacked the actor and stabbed him with a poisoned dagger.
The other priestesses began chanting a prayer, and I guess at least one of the players assumed they were casting a spell because he immediately declared he was going to shoot them. All of them. And then proceeded to mow down ten innocent women. Of course, the priestesses were the only people who may have been able to cure the dying actor of the poison.
I can't tell you how it ended because we haven't finished the adventure yet. Despite the blunders and horrific slaughter, I think we all had a good laugh at it. It's been quite a ride so far - what I had planned to be a simple adventure where the PCs would have to take part in a play while trying to fight off assassins to protect the lead actor has dragged out for weeks. The party has had to fight their way to the theatre, befriend wild barbarians, foil terrorist suicide bombers. One of the PCs encountered a long-lost relative, another may have a princess falling in love with him, another has blindly walked into and around a secret plot to overthrow the government and hasn't even realized it. And the curtain still hasn't risen to start the show!
The best part? I didn't plan any of this. I'm either a much better game master than I thought, or I fucking suck because this adventure should have been finished weeks ago...