The good news  
 
Had a half a tank of petrol but prices were low so thought I'd top up because it's likely to spike in the next few days.
I use the OTR app because I don't have to go into the store and you just scan your pump fill up and off you go.  Also normally I get 2-10 cents off the displayed price.  More often 2-4 and only once 10 cents.
Went to pay and it came up and said you've just won a free tank of petrol
The bad news - 

I rarely fill up a half a tank - why did it happen that day and not when my tank was nearly empty