I was working for the leading print marketing company that was starting to have serious competitors and post-IPO we were really concerned that people were going to order business cards from us all the time (since they were almost free) to track our daily internal sales via order number. I can't say that we ever had hard evidence that this was happening but we thought we had suspicious activity once so we changed it.

An aside, changing order number to a hash value can be a huge pain in a manufacturing environment when people have (mis)used it to for a variety of purposes beyond order identification.