We do a similar thing to mark documents that are still under embargo and one day I forgot to remove the watermark. The customer complained they had not received the document (a day or two after I sent it), and much searching was done. Finally we found a line in an outlook log on their end that said that the watermark was the reason the file had been put in indefinite quarantine until someone would take a look at it.

That's when I realized my error, and of course it was soon fixed afterwards. It is the first proof that it worked though, so I'm kind of happy about that.