Landing lag glitch
The landing lag glitch, also referred to as recovery carryover lag or RCO lag, is a glitch in Brawl and Smash 4 that causes certain characters to suffer lag under conditions that should not cause such lag. Specifically, certain special moves that place the user into helpless can have their lag "saved" if the user grabs an edge after using the move, but before landing. The next time the user lands on the ground, even if doing it by jumping from the edge or being hit by another player, they take the lag that would have been applied had the character landed in helpless. The only way to avoid the lag is to drop from the edge and jump back up without grabbing the ledge again. Of course, such lag should not happen because the character does not land in helpless, and such is the glitch - the game doesn't realize that it should forget about applying lag upon the next landing if an edge is grabbed.
The glitch cannot be avoided with specials that do not cause the character to become helpless and never disappear until landing on the ground; however it can be "overruled" by using an aerial that has landing lag, though with 35-40% more landing lag than would usually get for landing with that aerial. Preferably, one would use an aerial with lag to partially override the glitch, but still suffer more lag than the aerial would provide.
Characters affected by the glitch
Except for Donkey Kong and Bowser, who both get a relatively small extra two frames of lag, all characters listed have 15 frames for soft landing. Hard landing values are listed below.
In Smash 4
While the glitch no longer occurs after grabbing a ledge, most characters suffer a varying amount of landing lag if they are hit after the execution of their up special moves, but before they land. Additionally the amount of lag suffered is now equal to the animation length of the landing animation for the character, be it from hard/soft landing or an aerial. Captain Falcon, Ganondorf, and Zero Suit Samus also remain affected by RCO lag after they connect with Falcon Dive, Dark Dive, or Flip Jump respectively and land on a platform. Any aerial used in an attempt to override the RCO lag will also experience increased landing lag, like in Brawl, while if the aerial autocancels the RCO lag is experienced. However in some rare cases, such as Captain Falcon's up aerial, the landing lag received can actually be slightly less than the RCO lag they would have received otherwise. Some moves, particularly multi-hitting electric moves like Mewtwo's neutral aerial or Pikachu's forward aerial, induce extra landing lag on the character they hit if they end prematurely.