Bets are retrospectively void so my 'what-if' figures were normal in the moment and there was no way for me to know my liability was so huge.
These bets were to hedge an existing position so turns out I was actually gambling on the market all along without knowing!

Of course this means I also had the chance of winning, but you know, sod's law and all that

I can recall being significantly stung by an outage before on a Saturday afternoon but in theory I could have managed my position on a different exchange/bookmakers. There was also a time a market settled whilst I was mid-trade and my account balance turned to -£0.01, which meant I couldn't close my position. I actually found that so funny that by the time I realised what was going, and the race was already In Play, the odds would have changed dramatically anyway by the time I deposited so I just sat and watched the race. That one did win fortunately! Definitely one of my highlights from trading.
We have to expect + be prepared for mistakes but sometimes it's really not our fault!
Anyway it got me thinking...
What's been your most costly mistake for which BF were responsible?