I accept these mistakes can happen. They are not ideal but unless you make sure you read every label in detail and pour it yourself, life contains carbs.
It is annoying but as long as it is only a one off, it is not going to have huge impact in the longer term.
If you are suspicious in the future, I have heard of people using test strips to test their drink.
That said, I find artificial sweeteners taste sweeter than natural sweeteners so it is hard to tell.