[Demo Account - position closed at a non existing price?]

Take a look at this chart (1 min interval).
The vertical line is the time the short position was opened and the horizontal line is the stop value set and the exact price it executed at 17 mins later.

Is that "slippage"? Why did it even trigger?

[Demo Account - position closed at a non existing price?]

I'm only using a demo account (spread bet), but I just had a position closed at a price that doesn't exist.

Short position closed with, Time=13:01 Tesla Motors Inc (All Sessions) Buy £0.50 per point @ 21532

buy price peaked at time=11:18am @ 21517 and has been lower ever since (position was opened after this anyway)

 

Can anyone explain why this happened?

 

I would have to understand this if I was to put real money into this account!