Box Freeze

It's actually a bit easier to freeze the box on BO2 than BO3, but on BO3 it's more useful. Box freeze was first discovered in ~2020 using the death machine on BO1, but earlier this year pitshoot found out it could be done using the P.E.S and even claymores, prompting a series of tests to find out how it can be done on every game.
On WaW it can be done only by timing a switch to bouncing betties (meaning it can't be done on WaW Nacht)
On BO1 it can be done by timing a switch to claymores or by spamming the pickup trigger while holding a death machine/george waffe or while putting on/taking off the P.E.S mask
On BO2 it can be done by spamming the pickup trigger while holding claymores/buildables/shield/time bomb or by timing it with a grenade
On BO3 it can be done by spamming the pickup trigger while holding a death machine/specialist/shield or by timing it with either a grenade or trip mine
It's not known if there's a method on any other zombies games, though it seems people have tried extensively on BO4.

This can be exploited on maps with firesales (or using the hacker on Moon) to allow you to get 2 of the same weapon, by first freezing 1 weapon in 1 box on a firesale and then getting that weapon from a different box and upgrading it, then picking up the frozen weapon, allowing for 2 ray guns; however, the game does check for limited weapons (weapons only 1 player can have as opposed to all 4 players) existing in other boxes when rolling a weapon, so you can't do this to get 2 thunderguns or 2 wave guns; alternatively, you can use this to your advantage by freezing something limited and unwanted during a firesale (like the ballistic knife or crossbow) and then hitting the box for the weapon you do want, or after trading your QEDs for gersches on Moon, you could hit the box again with the wave gun ammo you have for QEDs and freeze the QEDs in the box to pick up later, as opposed to hitting the box for QEDs with 0 wave gun ammo later.

On BO3 this is extra useful, as if you freeze a box during a firesale, zombies stop dropping firesales as part of the drop cycle, meaning more instakills/death machines/max ammos.

1 thing to be careful of, however, is that if you move the box to a location where the box was frozen open (unless it was frozen open by hacking it on Moon, in which case this doesn't apply), once you pick up the frozen weapon, the box disappears from the map. This could be exploited on "Five" where the box is useless after the early rounds to get rid of 1 extra entity for the reset time, but it could otherwise ruin a game of Ascension, Call of the Dead, or Shangri-La.