Yep, the guy that stopped Sylvia Seegrist (1985) by grabbing her rifle thought it was a Halloween prank and he didn't think it was funny. He didn't know it was a real gun and that she was really killing people.
http://news.google.com/newspapers?ni...g=6635,6773402
People thinking the shots were firecrackers...
http://news.google.com/newspapers?ni...pg=994,1229123
http://newsok.com/three-killed-in-ok...rticle/1924230
http://www.msnewsnow.com/story/17292...3-hospitalized
If there is going to be confusion of concern for the participants, then that confusion almost always is with the intended and potential victims. The killer(s) knows why s/he is there, what the goal is, and what tools are being used to accomplish said goal. Most know to expect the resistance at some point and often have plans for it. The overall "plans" may not be great or well thought out, but they have a lot more of an idea of what is going to go on than anyone else there when the shooting starts.