As it was one of the more spectacular, I'd like to point to the Incident at Venlo, Netherlands at 9th December 1939, which severely crippled British intelligence and created excuse for the invasion:
On September 3, 1939, Britain declared war on Germany. The Netherlands, a neutral state bordering the Reich, was now on the front line of the espionage war. Acting on the orders of prime minister Neville Chamberlain, Capt. Sigismund Payne Best (British SIS agent) embarked on a dangerous and, as it turned out, disastrous game: over the next few weeks, he opened talks with a group of Germans who claimed to be army officers plotting to bring Hitler down and end the war.
In reality, Best’s negotiating partners were top SD operatives playing a game orchestrated by Reinhard Heydrich from Sicherheitsdienst (SD - the intelligence agency of SS) and authorized by the führer himself—a game that would end with an embarrassing, violent incident in the tiny Dutch border town of Venlo. That incident resulted in Best and another British spy spending the rest of the war in a series of Nazi concentration camps. It also destroyed Britain’s continental spy network in Europe and virtually eliminated any chance that Germany’s genuine anti-Nazi resistance would ever receive help from Britain.
While going to the meeting with "disgruntled, anti-Nazi generals", British agents (Capt. Payne Best and Maj. Richard Stevens ) were spectacularly kidnapped by the SD barely few meters from the Netherlands border. They were interrogated and as result, the British spy network has been crippled:
It remains unclear just how much information Best and Stevens gave away to their SD tormentors during the first days of intensive interrogation. In his 1950 autobiography Best is guarded, and implies that he kept his inquisitors at bay with skillful verbal dueling. But he and others concluded that the more inexperienced Stevens had cracked and told all that he knew. Incredibly, when captured, Stevens had been carelessly carrying a list of all of his agents—uncoded—in his pocket.
The presence of a Dutch agent (Lieutenant Dirk Klop, accidentally killed during the kidnapping) on the meeting gave the official prof that Netherlands is cooperating with Great Britain, which was later used as an excuse for the invasion.