Was recently on holiday for a month so I have a few.
First, I got a ticket fine on the bus from Orly Airport to central Paris. I bought a ticket but I didn't validate it on board. Had to pay something like 30 euros on the spot (on top of the 15 euro ticket I bought). I've tried to fight it with the french bureaucracy to no avail.
Second, flew into Bangkok from Australia for just two nights. I hadn't really thought too much about it and hadn't organised any cash. Was planning to get the train from the airport to my hotel but my flight was delayed 4 hours so I missed the last train and had to get a taxi. The taxi kiosk at the airport was pretty modern so I (incorrectly) presumed taxis would accept card payments. Anyway, ended up having to be driven to a random ATM by my taxi driver at around 12:30am to get cash out to pay for my fare.
Third, getting a taxi to Athens airport at 5am I was determined to not fall foul of the same mistake I made in Bangkok. The night before I left I asked my hotel front desk if they'd be able to call a taxi in the morning. I also asked if I needed to pay using cash or could use card. They assured me they could specify for a taxi with a card machine to pick me up in the morning. I repeated my request the next morning. When we got to the airport I asked the driver to confirm he took card, he gave me a resounding no. Leaving me in a mad rush to find an ATM in the terminal while the taxi driver had my bag.
It does seem that my weakness is transfers to and from the airport which is a bit embarrassing given my username and line of work.