Ive had that happen a few times and it is pure panic.

One important fact of having a lot of stuff listed with very little space for organizing is that the longer it takes for an item to sell, the more likely it is to get misplaced as stuff gets shuffled around.  Another important fact is that the more stuff you have sell in a short period of time, the more likely it is that you will forget to annotate it as sold in your record keeping system.  For me, I have a directory in Turbo Lister for each month and each account for Sold / Will Sell.  I've noticed that in the few instances where it turns out an item sold that had been previously sold, it was due to getting distracted and not immediately moving its entry to that directory.

Now, when something sells, and I suddenly get the nagging feeling that this could have been an oops, I first check the month that its pictures were uploaded to Photobucket (I make a new directory for each month there).  Then I go back to the associated spreadsheets for that month and later and see if it did indeed sell.  If it didn't, then I go back to the first spreadsheet and look to see what else was listed the day that item went up the first time.  That will quite often give me a good inkling of where to look, because chances are, one of those items will be one I had recently seen while dealing with other items.