When one is instructed to fast for 12 hours prior to blood tests, how strict is it and the relevance of what and when something is eaten during that time?"?

In other words – can I have a beer or a glass of wine exactly 12 hours before? Can I have a glass of tea 6 hours before? What if I forgot and ate a snack 6 hours before – you get the picture.

Thanks, Docs/smarty jellies who know this from experience.