Capedash Overflow is a glitch that allows a player to refill their magic bar by cape-dashing through an anti-fairy as their magic bar is about to fully drain.
To perform the glitch, a player must be cape-dashing in order to be vulnerable to the magic drain effect of an anti-fairy while wearing the Cape. Then the player must be dashing through an anti-fairy while they have a magic value such that the magic drain from the anti-fairy would go to a zero- or negative value. This overflows the magic value to 255 (an invalid magic value). If executed correctly, the Cape will remain on and the magic bar will start displaying glitchy graphics until it returns to a normal value.
While the player's magic value is invalid (above 128) they will not be able to use other magic items, unless the use of that magic item would return their magic to a normal value, and so the player will need to continue draining their magic bar using the Cape or by taking additional magic drain from an anti-fairy - until the magic bar is solid green again (indicating that the player has a normal magic value). Otherwise, the player may also simply collect a magic decanter of any size to return their magic value to normal. Green and Blue potions will not restore the magic bar to a normal value when in this state.
This glitch can be useful to refill your magic bar such as before boss fights that require heavy use of magic, provided there is an anti-fairy to set up on, for example the anti-fairy two rooms north of the Kholdstare drop down or the anti-fairies in the tile room of GT (provided you don't have an additional small key or otherwise can't grab the full magic decanter one room to the east).
The speedrun of the 2020 Festive Advent CalenDOOR features several uses of this glitch in order to save time by avoiding using a potion to refill the magic bar (which would otherwise cause 1.5 seconds of delay as Link's movement is arrested while the magic bar is refilling).
This glitch can be used to "extend" your magic bar for the use of particular items (for example to be able to use the cape for a longer duration than normal, perhaps to get through all of Spike Cave without taking damage, using a potion, or having half magic). You are able to use magic items as long as the use of the item would result in a legitimate magic value after the magic use (so a final value between 1 and 128), meaning for example you could overflow your magic value in order to get an additional use of fire rod than you would otherwise be able to do.