Not knowing where you are staying:
In Addis Abeba, I arrived knowing I would be met by my hosts. Customs asked me for the telephone number and address. I didn't know even the format of the phone numbers there. To make matters worse, Germany had deported an Ethiopian on the same flight and I was next after customs had to deal with them. The guy behind me handed me a note with a phone number. I gave it to the customs guy and was immediately cleared.

Customs-In-Training:

I arrived at CYQG on a private flight to pick up a pax for a meeting in Toronto from DTW. Usual procedure is to inform tower that you need customs and go where they tell you and wait. Never had an issue with waiting on the Canadian side but this time it took forever. Fired up the radios and asked if they forgot us? A few minutes later seven customs agents and a dog came marching up to the airplane. I handed out the documents got asked a few questions, then asked if it was ok to let the dog inspect the airplane. Be my guest. Then I realized that 6 of the customs agents were rather young and one who stood back was grandfatherly. After the dog ate my subway sandwich, to the chagrin of all the customs folks, I smiled and asked how long they'd been on the job. First time to inspect a GA arrival. They even offered to buy me a new sandwich. All in all, it was more fun than hassle once I realized they were in training.

US Geography Challenge:

After I moved from NM to MI, I drove over to Windsor from Detroit for the day. On my return, passport in hand, I got the royal questioning. The agent couldn't quite figure out why I was driving a Mexican car from Canada to the US. While my US PP had Mexican stamps, the car had NEW MEXICO USA license plates. I told him that the car was registered in the US and I had just moved from New Mexico, he told me I couldn't just bring a Mexican car into the US without permission. The car was actually made in Germany.