This isn't a problem. If I'm criticizing something that is the way they do something in that style, then I can be corrected. Someone correcting me means that I learn and others learn because I stated what I saw and understood.
When I see someone doing something wrong or a little different, I like to take the approach "Hey, we do that like this, I noticed you did it a different way and I was interested in why..." This usually leads to a good discussion of the differences, and neither side has to be defensive. Many people will be honest and respond with "well, I am supposed to do it better on this part, but I am still working on it..." In my experience, I have had better discussion and learned more, than when I pointed out "Hey dude, you are doing it wrong."
One thing I learned, once I start teaching, is that people learn differently. They start at different points, learn through different methods at different rates. I have had students who can watch and do. I have had some where it takes a long while to even approach the thing they saw. When teaching a new technique, most people can learn it 50% in a class. Then over the course of a few weeks practice, they get to 75%. They work on the that last 25% for the rest of their lives. There are many people out there, that will hardly get 1% in that first class. They will spend weeks and months trying to get to 10%.
Most people can at least fake: get off the line, brush block, off balance and reap foot. I have had students where it will take weeks just to get the starting stance close, and weeks more to move the correct foot and hand first. Then we start working on where those parts go. It is really frustrating when you have a student like this, work very hard to get the first 10% of a technique down only to have some do gooder step up and show them first how bad their technique is, and then show them an entirely different version of that technique... which this student has no chance at getting. The added confusion means a longer road for the student to learn the first technique.
Sure, we could have coordination tests for prospective students... but I am not sure I could pass one of those
The problem is that these issues are not only for the new guys who don't know their right from their left. Sometimes other arts, do things in different orders, or different emphasis. In my other training, I have worked hard to get things into muscle memory, so they happen without thought. When studying a new art, it has been very hard for me to do things counter to the patterns I have already learned, in order to do the new art.
In both cases, what is needed is a lot of repetition. Each time, work on the next parts that will make the biggest change. Thats where a good instructor comes in. He knows the student, and hopefully, knows what next changes will be most beneficial to that student. Each student, may need to work on different things in a different order.
In each of these situations, approaching that person with "Hey I am interested in why you do it that way..." allows for an open discussion. Knowing where people are coming from adds a lot to why they are doing it that way. I also think you can help them out better as well. There may be a principle or trick you know in your version (or even in another technique you know) that will help the other guy out a lot more, once you understand where he is. He will certainly listen to your advice a lot more than you if started with "Dude that is so wrong."