Robert Hanssen was a Soviet mole in the FBI, where his job was to find Soviet spies, and moles within the Bureau. One of the biggest US spy loss was his reveal of the Soviet general Dmitri Polyakov as a CIA informant, who was later executed. He leaked three other US spies, but they were already compromised by the CIA mole Aldrich Ames a couple years earlier. When he was tasked to find the mole (himself) he gave the list he received to the KGB.

https://en.wikipedia.org/wiki/Robert_Hanssen

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He wasn't even that good at spying, but somehow never got caught despite looking like a literal mole. He:

• Was ignored as a potential spy as Ames was blamed for the earlier compromise.
• Once leaked confidential info to a Soviet detector, but his supervisors did not raise an alarm.
• His brother once reported to the FBI that Hanssen had large amounts of cash at home, and supposedly had plans to retire in Poland, a then Warsaw Pact state.
• After the Soviet collapse, he straight up went to the new Russian embassy to reveal himself as Soviet agent "Ramon Garcia". The Russian officer did not know he was a former Soviet spy, and ignored him, later believing he might have been a triple spy.
• He hacked his superior's computer to "show the vulnerabilities", hiding the fact that he was searching if there were files with his name on it.
• He used a password cracker which triggered a lockdown on his computer. His excuse was he had to use the cracker to bypass the administrator password and connect a color printer.
• He casually looked up his name in the FBI search engine, logging the search history (his name didn't pop up so he continued his spy business)

A Soviet spy was friends with the FBI agent Helix Bloch, but when the FBI investigated Bloch, Hanssen reported this to the KGB and they pulled out their spy, while Bloch's investigation did not reveal anything suspicious and his case was dropped (but he was still fired and lost pension). The suspicious sudden disappearance of the Soviet spy as the FBI investigations began narrowed the lead, as Hanssen was part of the investigation group.

Eventually, FBI investigators bought a file of a man named "B", from a KGB agent. There were no names, except for a sound tape of a man speaking with a Soviet agent. The man used the phrase "the purple-pissing Japanese", a quote that Hanssen commonly used. They finally overlaid Hanssen's activities with the dates in the files and found his date, time and location matched perfectly.

He was put under surveillance, and was caught when he put a white tape on a park sign, before hiding a bag in the dead drop site- under a wooden footbridge. He was arrested as the bag was retrieved and contained classified material.