UK to Netherlands
In the 1980s I used to swap files with a friend in The Hague by posting floppy disks. Posting to The Hague was fine but in the other direction the UK customs opened (and resealed with a note that they had done so) every floppy coming from The Netherlands. We had no explanation; all we could think of is that they were looking for pr0n. The customs guys must have been pretty mystified to find Excel files with directions and distances for our next motorcycle tour of France! I was pretty pleased when we got email!