In Germany, there's one that says you can drink beer and then wine, but not the other way around, because that would make you sick.
...which is stupid and just not true. It's irrelevant in what order you drink beer and wine; if you drink too much, you'll get sick either way- if you don't, you'll be fine no matter what you drink first.
I heard that one for the first time when I first came to Germany a few years back (so I've got no idea whether it's an international urban myth).
No matter how many times the thing gets disproven on the telly, radio and whathaveyou, people are reluctant to let it go. It's annoying.