Currently, in the ESEA League rules, players are forbidden to "pixel walk". This is defined as when a player is taking a position on an invisible ledge which is not within the design of the map but where the player is otherwise stuck. At this point, if a player takes a pixel walk position and has any impact on a round then it shall result in a standard infraction as per the League rules.
A clear example of a pixel-walk is available below where the player providing the boost is stuck in mid-air.
Confusion sometimes arises when players stand in areas where they do not appear to be stood on anything. The key distinction, however, is that when the ledge is part of the map design then it is not a pixel walk.
This is not a pixel walk because the player is standing on the window ledge, even though at this angle it does not appear anything is directly below the player model.