The Tangled Web Author:Philippe Ganier-Raymond From an Amazon.com review: — This is an excellent book on the history of Allied agents infiltrated into WWII occupied Holland from England. — Each agent was given a "security check" -- a "mistake" the agent was given to include in each morse radio message back to England. The absence of the "mistake" would signify to England that the agent had bee... more »n captured and was sending messages under duress.
In March 1942, agent Hubertus Lauwers was captured and forced to signal under duress. Following his training, he omitted his security check. England failed to notice the omission and continued sending new agents to meet Lauwers. Each new agent, of course, was captured. Lauwers even risked his life to send the word "CAUGHT" and on a later occasion "WORKED BY JERRY SINCE MARCH SIXTH."
The British, through negligence and over-work, remained oblivious to these clear indications that Lauwers had been captured, for two years. All agents or information sent to him were captured by the Germans as well. This resulted in the capture and death of dozens of agents and partisans.« less