I think that if you are an SO, especially if you are at a major match, before you call a penalty you need to know for sure whether or not something is an infraction, and, what the proper penalty is. Sometimes it's a judgment call... at a major match in 2008 I had two calls made on me on the same stage. One was clearly in the rulebook, and the SO made a mistake, and the other was questionable. The clear violation was the stage description said fire two to the body, 1 to the head. The rule book says that any hits in the head will be called -0 unless it is specified in the stage briefing that shooting all shots to the head to avoid sight transition would result in a procedural penalty. I fired a makeup shot so I had four shots in the target - two in the head, one in the center mass -0 and one in the -1. The target was called -1. I pointed out to the SO after that since it was not specified that it would be a procedural to have extra head shots the target should be -0. He offered to change the score sheet but I declined because at my skill level 1/2 second added to the match would not make a difference. The second violation was an allegation that my magazine carrier was too far forward of the centerline of my body. The SO said this was a procedural, then moved my carrier maybe 1/8" rearward and said this was where it should be. That's a judgment call - if it's that close of a call I say give the benefit to the shooter. The SO should ask him/herself two questions: 1) is what the shooter did unsafe? and 2) did the shooter gain a competitive advantage by his/her actions? Moving the mag carrier 1/8" inch rearward (seriously, it was such a tiny adjustment) in my opinion did not give me a competitive advantage but that's a judgment call.
I was at another sanctioned match this year where someone was airgunning and the SO first stated that he was going to give the shooter either a "stage disqualification" or a Failure to Do Right (FTDR-20 second penalty). The MD was called and the SO was advised that there is no such thing as a "stage DQ" in IDPA and a FTDR is given. Problem is, the rulebook clearly states that airgunning is procedural (3 seconds). The shooter in question has a cloudy reputation for gaming and pushing the rules envelope. If what he did makes you think he was "unsportmanlike conduct", "an unfair action", or something that "tends to make a travesty of defensive shooting" then call it that and assess the FTDR. All of those quoted phrases are taken out of the rulebook describing a FTDR. If what he did was airgunning then it is 3-second procedural. So I guess the right thing to do is if you are not sure (we can't all memorize the rulebook) then look it up before assessing the penalty.
I was one of the people Trav called about this... my first reaction, and the first reaction of several SOs that I spoke to was that a bad call was made. Turns out we were all wrong - the permitted modifications list in inclusive, and, an extended slide stop is not included.
On one hand, I commend the SO for spotting something that we now know is a clear violation of the equipment rules. He spotted it when SOs on several stages, including the ones doing the equipment check and chrono, missed it. On the other hand, if you are not sure that something is a violation, and, if you can't point it out in the rule book then you shouldn't be making the call. The SO did call the MD and asked for a ruling. I can see where the SO at this major match needs to move things along but I don't see why the MD couldn't have pointed out the rule. I think it would have been better if the SO told the MD that he thought this was a violation and ask the MD to research it while the SO continues the match. I believe it was wrong for the SO and the MD to say it was their "judgment" call. Positioning of the mag pouch behind the center line in the example above is a judgment call - this equipment violation is in black and white.
Trav, I know it sucks to get the DQ and spoil your match. As someone who has volunteered to SO at a few club matches and sanctioned matches I can tell you that sometimes it's a thankless job. Sure there are some "range nazis" out there and folks who seem to take pleasure in calling penalties and DQ'ing people but I think they are the exception and not the rule. I encourage you to keep playing the game and going to sanctioned matches - they are a usually a cut above the typical club match and worth the time, money and effort.
My $0.02