Envelopes. Namely, we print envelopes with bleed, and need laser proofs with crop marks so that the pressman can see the correct position of the artwork. But for ripping, the PDF has to be to size (Or size plus bleed), so there can be no marks in the file. Thus, not being able to print accurate crop marks out of Acrobat leads to 20,000 envelopes with the logo 1/8" too tall.
On a philosophical note, I hate, hate, HATE, crop marks in PDFs. Partly because they're almost never set outside the bleed, but mostly because the marks on the press sheet will be from the imposition software, so there's no point in having them in the PDF. All a PDF should have is bleed, maybe slug, and a trim box, and not the ten tons of crud that end up in the file when people decide to click on all the little boxes, and I remove with Pitstop every single time.
Sorry, it just bugs me.