Not really slippage, but this story is from 98. I entered an order on ARBA at 108 when I was using "ETrade" for daytrading (first mistake). The stock was falling, falling, falling, 102..No confirmation of my buy. I figured well shit, I guess I got filled on the buy and it's falling and since I can't see anything for confirmation, I will go ahead and put in a sell as I saw it rising back up to 106. Now both orders are in and 20 minutes goes by, still nothing. So I call and guy says, orders never went to the market, want me to cancel them? I say sure, what the hell.
So a week goes by, I wake up to a 8K loss on the unrealized profit/loss and see that they reinstated my sell order (seeing it as a short), but didn't reinstate my buy order. So I call and ask, what the hell? They said, one was limit, one was market. Can't cancel market orders. Price was at 120 or something at this point LOL..after two weeks of going back and forth, I got it all straightened out, and then moved to Castle direct access. It was all so comical how screwed up things could have gotten back then.
Anyway, it's my story.