From Clive Stafford-Smith’s book about Guantanamo and the detainees, Bad Men: Guantanamo and the Secret Prisons (extract shamelessly nicked from another source on the internet):
Yusuf was technically a citizen of Chad, but he was born in Medina, in Saudi Arabia, and had lived his whole life there. The US military could have got his birth certificate from their Saudi allies with a telephone call. It was a little more difficult for me, but I got it and it showed he was born in November 1986. After four years of intensive interrogation, far from securing a rich harvest of “enormously valuable intelligence” (General Geoffrey Miller’s words), the military could not even work out Yusuf’s age, just as they could not spell Binyam Mohamed’s name after years of torture in Morocco and Afghanistan.
[…] Yusuf explained how his [initial] interrogation quickly descended into farce. Early in his captivity the US agents questioned him with the assistance of a translator who used a dialect of Arabic in which the word zalat means money; in Yusuf’s Saudi dialect it means salad, or tomato. Yusuf reconstructed the interrogation as best he could remember it.
“When you left Saudi Arabia for Pakistan, what zalat did you take with you?” demanded the translator, suspecting that the money must have come from al-Qaeda sources.
“What? I didn’t have any zalat when I went to Pakistan.” The 14 year-old was confused. He had been through a difficult time since his seizure by the Pakistanis. He was prepared for any trick the Americans might spring on him, but all this talk about tomatoes was beyond him.
“Of course you had zalat. What do you take me for? An idiot!” The translator flared into hostility.
“I didn’t! Why would I?”
“Of course you did. Now tell me, where did you get the zalat you took with you?”
“I didn’t take any zalat with me. I didn’t!”
“Aha! So you got zalat in Pakistan when you arrived?”
“Well, yes, what zalat I wanted, I could get there. That’s natural.” Yusuf was trying to be conciliatory, though the conversation continued along this strange line.
The translator seemed suddenly excited. “Where could you get zalat in Pakistan, then? I want a list of places. Details. Descriptions, places. Details.”
Yusuf wanted to keep him in a good humour. Trying to remember Karachi, he began to discuss places in the market where one might buy salad. With each description of a market stall the translator turned to the American interrogator, who took careful notes.
That evening Yusuf was returned to the cage where he was being held. He was a very muddled adolescent. He talked through his bizarre interrogation with other prisoners, turning over each of his recollections.
Finally one of the older prisoners solved the puzzle: “You were talking about tomatoes. They were talking about money. That’s what it must have been.”
Arabic isn’t a monolithic language; while its written form is somewhat standardized, it is split into many regional dialects, some of which are mutually unintelligible, effectively different languages.
The unfortunate 14 year old Yusuf got himself sold to the US intelligence services by a Pakistani entrepreneur who saw the “reward paid for Al Qaida members” ads, read them as MAKE MONEY FAST, and grabbed the first passing stranger. And the US military, being extremely short on skilled Arabic translators, assigned an interrogator who spoke the wrong dialect and didn’t know enough about Arabic to know how wrong they were.