I was trying to check out and the items I had disappeared from my cart, what happened?

I was trying to check out and the items I had disappeared from my cart, what happened?

Due to the nature of flash sales which have an incredibly high demand and limited inventory items are not reserved until the transaction is complete. If inventory is down to a single item and multiple customers have it in there cart, the customer that completes checkout first will get the item and the others will get a message saying that the item is no longer available.