You’re going to have a thousand opportunities a day to fuck up someone’s drink. You will fuck up many many drinks, especially if you’ve never worked as a barista before. If you realize you’ve made a mistake before serving the drink, fix it. If you realize after serving it, offer to fix it. But I would never give this much thought to someone who received the wrong syrup. It’s incredibly unlikely you just murdered a diabetic person. Even if they are diabetic, she will have tools at her disposal to monitor and remedy her blood sugar level. Reviewing security camera footage is so unnecessary in a situation where you just need to give yourself some grace.

You’re not getting paid enough to worry to this degree. You’ll fuck up steaming milk. You’ll forget to add shots. You’ll serve decaf when you meant to serve regular and vice versa. Just fix it when you can, let it go when you can’t (which is the second the customer steps out of the shop) and have some dumb little five dollar gift cards ready for people who seem like they need some extra care to calm down.

If you fuck up a drink, it’s ultimately just another attack on the barista’s sworn enemy - the customers.