Driving home after Christmas and ordered a Tonic water with a pub lunch, specifying that it must be slimline or sugar-free. When I checked the bottle it was this trendy (expensive) Fevertree "Naturally Light" one. I had an usually high reading later, so I checked the details and they came out at: "Total Carb: 3.8g of which sugars: 2.9g", the sugar content being attributed to fruit sugars.
Just goes to show you can't let down your guard for a minute...