The plugboard of the enigma machine provided a false sense of security to the Germans. The sheer number of possible combinations, 10,000,000,000,000,000 is a daunting number, however, the plugboard itself can be isolated and broken by frequency analysis. The most complex part of the enigma machine is the scramblers, placement of them, and the their settings. This only has around 100,000 different combinations, a completely possible human task to handle; way easier than the 10,000,000,000,000,000 combinations of the machine. The hubris of the Germans to believe their machine was impregnable allowed them to get lazy when sending messages, like repeating the codes twice. The Germans had no idea that repeating the codes twice would create links that corresponded with an exact ratio to scrambler settings. The Germans were simply mesmerized by the large number of possible combinations.
Many times nowadays companies hire hackers to intentionally hack into their security and provide information on how it would be improved. Had the Germans done this, they would have realized that repeating the keys was a bad idea and allows for breaking the code. Being complacent allows for error, always believe that the other side is one step ahead, otherwise they will undoubtedly become one step ahead.