In Japan, and many other countries in this world, one should not directly hand money to cashiers in commercial exchanges. This taboo is fairly well known and has appeared in various media (including a great meme format) that most westerners that consume Japanese entertainment are aware of it. Despite this, the very first financial interaction that I had in Japan, I tried to give the clerk money like I am at an American grocery store or fast food restaurant.
I was buying a bus ticket from the airport to downtown Tokyo (Most travel guides recommend that you do this the first time you are in Tokyo as it is cheap and gives you a good view of the city), and I just, without thinking, handed the cashier the equivalent of twenty bucks. He looked at me funny, but did take the money from my hand. A bit later did I realize what I did and felt super embarrassed for way longer than I should have.
All cashiers in Japan will have a little plate that you set your money in. Always remember, put your money or card there. The clerk will take it and put your change or card back in that same plate. Which allows you just pick it up and go about your day.
Mistakes are inevitable, especially in unfamiliar cultures. As nerds, we think of ourselves as smart and that smart people do not make mistakes. We tend to stew on mistakes that we do make and this stewing can bleed into other experiences, ultimately leading to lessening or even ruining them. I am hoping that by looking at my mistakes, you will look at yours and realize they are not really that bad, and will help keep you from ruining other memories.