When US people are visiting Aussieland they often seem to be unaware at first that we don't use US bills here, it's not "just like Mexico", and they frequently talk too loud. Not exactly a "custom"...just ways,lol.
The Dutch eat way too much cheese, lol. I hate cheese!!!
Not a fan of royalty - British or otherwise - so that applies to any country with a Monarch. For the British - congealed pig's blood...really? Ugh. Yuck.
Any country that implements Sharia law.
Certain sexual customs that certain tribal people have practiced/do practice around the world. I watched a video recently that described (in words) some of these. You don't want to know...revolting.