I hope Trevor won't mind me answering.
Certain constructs should only be used with across clues, like "to the west" which means to reverse. The equivalent down construct would be "to the north".
The one that Trevor mentioned was "look up", which would be silly for an across clue, for which the equivalent might be "look back".
The number of these constructs is limited only by the imagination of the setter, so it would not be possible to list all of them.
I try on my blog to point out, whenever I can, that the construct only applies to the appropriate clue.
(You can always find me by googling "Big Dave Crossword Blog" should you want to!!)