We (2 adults, 2 teens) spent about two weeks in Germany along the Rhine and Mosel. Rented a car for part of it but used trains a lot as well. Almost ended up with a fine because I rushed and didn't pay attention.
Frankfurt Airport to Bacharach - kiosk spit out one ticket (mini-group) and a receipt. TWO pieces of paper.
Bacharach to Koblenz, same. TWO pieces of paper.
Trier to Luxembourg City, same.
Luxembourg City back to Trier... I went to the kiosk. Train was leaving in 5 minutes! We have time! Two adults, two kids, yes, I want a receipt, insert card, grab two pieces of paper and rush! Whew! Made it!
I never looked at the pieces of paper. I assumed it was my family pass and a receipt.
Entering into Germany, the meanest looking gentleman from Bahn walked up and I confidently handed over my two pieces. Sure, I'll give him the receipt just to prove I paid.
His eyes lit up and he started barking at me in German. I have no idea. "Nein deutsch."
Turns out, I grabbed two pieces of paper, but they were only two of our four tickets. So I'm missing two. They are still in the machine in Luxembourg along with my receipt.
I politely explained my mistake and I even quoted him the amount I paid, to the cent. I think he realized I did actually pay (why would I try to cheat by only paying for half of us?) and said now in perfect English, most Bahn conductors would give an immediate 60E fine but he'll cut me a break by simply selling me two more tickets. Whew. I never handed cash over so fast in my life!
Point being, don't rush and pay attention to what the machine is spitting out!