It was a plan of "childish simplicity"
From the blog Futility Closet:
The Treaty of Berlin was drafted in secrecy, so its framers were astonished to find it published in the London Times. Journalist Henri de Blowitz at first refused to reveal his source, but at last relented near the end of his life. Well before the congress started he had attached a confederate to the clerical staff, but the man felt he was being watched, so the two could not dare to meet or talk. Finally de Blowitz noticed that they wore hats of the same type and color, and he hit on a plan of "childish simplicity."
Read how their devious plan was almost foiled at Futility Closet>>