Opportunity loss. When I worked IT in brokerages we always had to reimburse traders for missed opportunity from technology issues -- which was abused, whenever trades went against them they would claim their UI was unresponsive and we'd have a programmer scour the logs with millisecond timestamps to prove the UI did respond and how quickly, then cross reference with the tradebook to see if its a trade that would likely be executed immediately or not etc. Often times the resolution is that the trader clicked trade at this timestamp, the order was sent at this timestamp, the exchange acknowledged at this timestamp, but you tried to buy 10000 shares at X price and only Y were available, so the trade did not execute immediately or not at all.

Edit: Class action lawsuit was just filed for loss of opportunity - https://news.ycombinator.com/item?id=25945052