It's Thursday today, and I knew something was going down, as it were, as soon as I got to 18A: What margarine has, unlike butter (NOC[HOLE]STEROL), but I didn't yet know what, exactly, would be the trick until I got to the revealer: WATCHYOURSTEP ("Tread carefully!" ... or a hint to four dangers in this puzzle.) As soon as I hit it, the critical Down answers lit up.
So what we have, then, are letters needed to complete the Across answers that have fallen into a HOLE, a DITCH, a GAP, and a PIT. Two of the Down answers span words in the Across, two are contained within a single word. Honestly, I don't mind that at all. The trick works whether it spans words or not.
Loved the start of the puzzle today - 1A: Acrobat displays (PDFS). (Dad, Adobe Acrobat is the program that is used to create PDF files.) That's a lovely clue. Off of that I dropped in ShIP for "Berth place" ("You've a berth on board this very ship!"), but corrected it to SLIP before the buzzer. My error came over in the opposite corner, at the intersection of BOO ("Are you blind, ref?!") (Boo!) and SYOSSET (Long Island community bordering Oyster Bay) (never heard of it). I stared at that blank square for a good two minutes before guessing an A, and it was only after running the vowels that I realized that BOO must be right. That's not really an equivalent clue, in my opinion. Maybe the critique in the clue could be called a "jeer," but I think that a BOO is a BOO. It's not just any criticism. My Webster's backs me up on this, but perhaps an unabridged might have a more general definition of the word. BADRAP? You tell me.
Overall, I enjoy any puzzle with a trick, but I wished it hadn't been revealed, and there were quite a number of concessions (TOOLER, ULTA, ACEH, ENTS, etc.).