I don't drink fizzy drinks (hate the bubbles, feels like acid burning my mouth!!!) But I have had similar things happen in Starbucks as they do sugar free syrups, so in the summer I will get a sugar-free vanilla syrup iced latte rather than their frappés.
I try to always double check my order or say something like "please ensure it is the sugar free syrup" to make sure they've taken my order properly but I've still had a few times where they have used the regular syrup. I find that sweeter has a different kind of 'sweet' taste to sugar so I a couple of times I have taken just a sip and had my suspicions and then gone to check what they made it with or asked them to make it again, but one time it was only after I'd finished the drink and left the café and felt a bit 'swirly' that I realised the error.
I wish other high street cafés had the sugar free syrups though, I usually wouldn't go to Starbucks but I am not aware of any others that have them.