I'm using Acrobat 9.5 (Windows) in a business where we handle PDFs from customers every day. For each project, some of my clients prefer to send me all their pages in a PDF that has already been assembled. Other clients send the pages individually and I use Acrobat to group them, check the page order, and output an assembled PDF. (We convert the PDF to a series of images they can view online in a browser.)
Recently I started having problems with projects for a client who sends individual pages. Two months ago, after I posted his project online, he emailed to say one pair of pages was reversed, and could I please fix that. It was pretty embarrassing, considering how basic page order is. So, one month ago, when I combined the pages in Acrobat, I very carefully made sure the pages were in sequence. S-l-o-w-l-y counting "10 ... 11 ... 12 ... 13 ... 14 ... etc" as I viewed the pending combination in Acrobat. I saved the result and converted the PDF. This time when he contacted me to say THREE pairs of pages had been flipped, I was amazed.
I have a half dozen clients with up to 50 or 60 regular projects. This only happens to one client. Plus it's recently started. Can't find anything wrong with his original files. They have normal file numbers. Any ideas?