I guess I should start by asking the obvious - apologies in advance 😳 - any way the wrong slip was printed? Like it was someone else's order the first time around?
Otherwise, I can't really explain that or say I've experienced anything quite like that, more some very minor anomalies. I pick using a tablet - basically I look at every sold item (photos included), pick it, then take the days orders to my packing area and use the packing slip to allocate the items to the correct package. So far I haven't gone too wrong with this, save for a couple of minor hiccups where an item wasn't showing on ebay as part of the order (when I was picking in the stock room), but it was on the packing slip.
Probably completely unrelated, I also remember a couple of years ago, actually, I had someone place a relatively large order with me, picked and packed everything perfectly, or so I thought, until the buyer contacted me to let me know one item was missing from the package and the packing slip, (the buyer confirmed this, as I include the packing slip with the package). I checked their order, which did show the item, and reprinted the slip and lo, it was suddenly included. (I know the item itself was left out of the package as I had a photo of everything that was included for that specific order).
