If you've read the Book of War for Belegarth, you've heard of "medieval combat precedent". The phrase gets tossed out quite a bit during rules discussions, especially those between fighters from various realms/regions. Part of the reason for this is that the rules for Belegarth (and Dagorhir) leave several items open to interpretation. The rules are sparse to allow ease of use, which also means some rules aren't fully explained.
This leaves us with a several "grey" areas that some people play a little differently than the rules might suggest. The following list is by no means comprehensive, but covers a few of the most common. Check with your local realms and events to see how they play the them.
In no particular order:
1) "Shot in motion". According to the rules, when your arm gets hit (or your body), you drop what is in your hands. However, a large portion of the sport plays shot in motion, where you can finish any swing that was already started and have it count, within reason. The opposite approach is "atomic clock", where the swing is discounted immediately.
2) "Double tapped legs". Subsequent strikes to a disabled leg are ignored, unless both knees are down or you are rolling, etc. However, many play that you must be portraying the wound before that rule applies, meaning two hits to the leg while you are standing can kill you. You can avoid any confusion by taking the wound quickly (and wearing good knee pads...).
3) "Grappling". Armored fighters can't initiate grapples against unarmored fighters. Archers can't grapple. However, the rules don't mention grabbing equipment. Grabbing a spear shaft isn't generally considered a grapple. But what about a sword handle, a strap shield? There isn't much precedent, so I usually use safety as a test. An archer grabbing a spear to defend themself isn't placing the spearman in danger (6' away), but grabbing a weapon handle might place both of them in danger of the poky bits from the arrows and bow. My opponent can easily let go of their weapon to get out of the situation if I grab their handle, but not if I grab their strap shield.
4) "Head, then body". The rules say to take the worst of the hits of a strike hits more than one zone. But the head is an illegal target, so what happens when a shot hits the head, then hits body? Two sides to this one, either the "worst" means "bad for you, take the body" or "illegal target is worst, ignore the rest". Precedent, from back when, was that this is a body shot. Recently, more people air on the side of it being counted as a head shot. I generally try to gauge it based on how much of a head hit it was. Grazing a bit of an ear and landing solid on body, it's a body shot. Blazing me in the side of the dome and carrying a somewhat light hit down into the body, it's a headshot. This varies greatly from fighter to fighter. Best advice, avoid hitting people in the head.
5) "Late hits". See number 1. Your opponent throws a shot after he's been hit in that arm, so it doesn't count. But, you got hit and started dropping your gear. He says "late", implying you are still alive and should keep fighting. All fine and good. Though, none of this is in the rules. Normally, if you take an injury, the herald is the only one that can/should "heal" a wound, and even that is a stretch. However, "late" calls help keep people happy and fighting, and are generally accepted. Just give fighters nearby a friendly heads up that you are still alive. If you've been dead for a while, just stay dead, especially if being alive again would give you a very big advantage (behind enemy lines, etc) that you didn't already have.
6) "Point!". Guy throws a javelin, hits with the haft, then yells "point!" expectantly, thinking he has landed a good, point first javelin hit. The rules are honor based, you are in no way bound to his call of point, if it indeed hit with the haft. This call is something that probably should be replaced with "haft" or "nothing" when the thrower is pretty sure the hit wasn't valid, or "javelin" if someone is looking around to see what hit them.
7) "Pool cueing double greens". There was recently a large discussion about this one. Pool cueing is letting the spear/glaive slide through one hand while pushing with the other. Dagorhir specifically labels this as a one-handed stab, Bele doesn't specify. I call them as a single when I glaive, but others want to call them double. This is because the rules say "when wielded with two hands", leaving the amount of effort applied by the second hand undefined. Many play it like Dagorhir, where both hands have to be firmly gripping the weapon from the start of the stab until contact is made.
8) "Calling colors". While the Belegarth rules only specify that you need to call "two" when attacking from behind with a two-handed weapon, the general practice is to call the type of hit if it is anything other than a one-handed, swung weapon. Preference seems to be calling colors: red for class two and green for class three, while calling blue for one handers from behind. Also, calling stabs as "single" or "double" is acceptable, especially for polearms that switch between one and two handed often.
9) "acknowledging hits". Calling hand, armor, light, garb, graze, pierced arms, and any other call about how you got hit and didn't take a wound or die isn't required by the rules. However, playability says otherwise. Even if it is a few second afterwards, a call can help clear up the situation and prevent your opponent from getting upset that you aren't taking hits. As long as you are correctly taking your hits, missing a call here or there isn't the end of the world, just do your best to call what you can.
10) "Solid Shield Breaking Hits". Different places have different standards on how hard a class two weapon needs to hit a shield to qualify as a shield breaking hit, especially when adding in Dagorhir* realms. Some realms encourage the idea that it only needs to be as hard it would need to be to count against a limb. Most large events play it as needing a significant amount of extra power to break shields. This probably comes from the historical/practical argument of "would that shot have actually damaged a shield in a significant way". I prefer the heavier hitting style, on both sides of the hit. As a two-hander, I'd rather people only take "good" swings, rather than lighter hits that weren't intended to break their shield. As a shieldman, I expect other red users to be capable of the same hits.
*Dagorhir has a few extra rules towards discounting the hits that could easily be misread to mean "if it hits a weapon first, it doesn't count." As best I can tell, that isn't at all the intention, but I have seen it played as such.
Parting thought: bastard swords (min red) aren't really intended to be great for breaking shields. Try hunting armor instead.