@umpire16 Interesting discussion. Might I recommend just having a blanket statement? "If you are not a certified umpire, and/or all of your rules knowledge is based on watching MLB and listening to announcers, you are not permitted to argue".
In all seriousness though, this list could be exhaustingly long. Using yours, and thoughts that came to my mind:
1. Batters Interference - The only movement a batter is permitted to make is a swing. Any steps (towards plate or backwards), leans, falls, etc, make them liable for interference with a catcher. Even a follow through hitting the catcher is INT in high school rules.
- The exception is when a runner is coming home (usually on wild pitch), the batter needs to get out of the way if time permits, which it does unless it is a suicide squeeze situation.
2. Rather than limiting what is fair to "the base is fair", say "all bases, including home plate, and both foul poles, are in fair territory".
- I have called a home run on a ball that hit the foul pole in the air and the coach said "how do you not know that it hit the foul pole?"
3. Tie does not go to the runner on a force out - this sandlot rule causes many arguments on bang-bang plays, usually at first base...and let's not argue about "plays at first aren't a force out". I could go on a soapbox here b/c I know "it isn't", but when the batter hits the ball, by rule, they are forced to run to first but I digress
4. Speaking of force outs, how about the definition of what a force is? It is a situation put upon a runner, not the method in which they were put out.
- Had a friend umpire a tourney game this past wknd, bases loaded 2 outs, grounder to third baseman, who tags r2. Coach argued vehemently that it wasn't a force because he was tagged out and the run should count (r3 had crossed home before tag)
- While my example from this past wknd is more rare, the outs on appeals falls into this same category and is often misunderstood. Many think because you do not have to tag the runner to get the appeal, it is a force, which is incorrect.
5. You mentioned a mechanics issue...how about this one? The 3rd base coach has the best angle on plays at 3rd, in fact, he has the angle WE WANT but CAN NOT GET in 2-man. So stop arguing about close plays at third and our positioning b/c we have to take them from the working area.
6. "Runner must slide" - There is no rule in HS baseball that a runner must slide, at any base. They can not initiate contact or deviate to interfere or create contact, but they don't have to slide. Even umpires miss this, as I did NOT call a FPSR in a HS tourney last wknd bc the runner never impeded the fielder and the coach said "he has to slide" and I said, no he doesn't, that's a college rule, not HS, and coach said "last week we had 2 runners called out bc they didnt slide", and I said "not sliding makes them more liable for interference, but they still have to interfere with something to call it". So this a rule even amateur umpires misunderstand too, especially if they work in multiple rule sets.
These are just some things that have come into my head, I am sure I will think of plenty more later, but these are all things that tend to cause some issues more often than they should.