The following is based on my reading of the Rules
“Don’t Take that”
This appears 4 times in our rules, you often hear this call when someone is joking around and hitting friends, or when you accidently hit a friend and don’t want to take that hit. However below I’ll do what I can to point out that is not allowed by the Rules.
The only times you can tell someone “Don’t Take That” are as follows:
If you hit a participant after your Arm has been Taken, If the shot was illegal in anyway, Illegally Wielding Weapons, or hitting with the side of a thrust only Weapon.
Late Shots
Sometimes you will strike another participant immediately after receiving a hit that injures or kills your PC. For example, just before your sword makes contact with an opponent, your attacking arm is struck by another weapon. Even though you are incapable of physically halting your attack, the injury your PC received renders the hit ineffective. There is no "follow through" effect that allows your attack to be successful. This is called a late shot. If you deliver a late shot, it is your responsibility to inform your opponent to not take the blow. Common phrasing includes "Don't take that!" or "Late, on your arm!" Like other shots, you may not declare that someone else hit you late. If you think someone is failing to call their late shots, question them after combat ends, or bring your concern to a marshal.
Illegal Hits
Sometimes you may hit someone with a part of your weapon that doesn't actually damage their PC, such as the pipe of a weapon, the side of a thrust-only weapon (see Section 3.7: Weapon Rules), or the shaft, fletching or nock of an arrow or javelin. If you do this, be sure to call "Don't take that!" to alert them that it wasn't a legal hit, and that they may resume play as if the hit never occurred. A player may also call "shaft" if they received an illegal hit from an arrow or javelin, or call "slash" if they received an illegal hit from a thrust-only weapon.
Wielding Weapons
For the purposes of the rules, you are wielding a weapon or shield if you are holding it in your hand(s) and you attack, parry, or block with it. You are wielding a combination of weapons and/or a shield if you attack, parry, or block with either of them. Simply holding a weapon, or menacing with it, does not count as wielding unless contact is made. You cannot wield more than one item in a hand at once.
If you are wielding an illegal weapon or combination and you attack, then you should tell the target "Don't take that." If something you are not wielding blocks a shot, then you should treat the blow as if it had landed. If you are not sure where that blow would have landed, then assume it would hit the location that would cause you the most harm (e.g., an unarmored kill location). There are special restrictions for spellcasters in terms of wielding and blocking with different weapons, and specific consequences for doing so outside of the spellcaster’s restriction. See Breaking Weapon Restrictions for more information.
The size of a weapon dictates how it may be wielded. Weapons cannot be any smaller than 12", and cannot exceed 8' in length.
Thrust-Only Weapons
Thrust-only weapons cannot strike an opponent with a "slashing" or side-to-side motion. If you slash at an opponent with a thrust-only weapon, you must tell them "Don't take that." As long as the tip of the weapon strikes with a forward motion the blow must be taken. If you are not certain that a thrust-only strike landed properly, you must assume it was a slash and tell your opponent not to take the shot.