The only time I have messed up so far is not remembering what I have ordered. I purchased MP Acidstorm from Robotkingdom but I had a pre-order in the list so I wasn't going to receive it for about 3 months. In the meantime I forgot I ordered it and ended up getting one off eBay.

It took a few weeks before I realised I had already ordered it. RK were pretty good about it and simply cancelled that part of my order and refunded the money. I have a spreadsheet now I use to track what I have ordered so this doesn't happen again