It violated the primary rule any good reporter is trained to follow. Confirm your sources, and back them up with at least two independently verified alternates willing to attest to your original source.
Woodward and Bernstein made this type of Journalism famous in the "Watergate" scandal. The beauty of that story was how they painstakingly sourced their tips, and backed up their "leaks" with confirmation from official sources.
Where is that type of Journalism today?
Someone leaks a copy of Trump's tax return, and it's immediatley published.
Someone leaks a name illegally unmasked during a FISA warrant surveillance, and the innocent name is immediatley published.
Someone leaks parts of a private conversation between the President's national security advisor and a Russian diplomat and the conversation is published.
Someone leaks details of a conversation between the President himself and the Russian
Foreign Minister, and the conversation is published.
Someone calls up and reads from the notes of a memo written by the former FBI Chief between himself and the President and the details are published without the publisher ever demanding to verify the contents of the memo.
This isn't Woodward and Bernstein style investigative reporting, this is a media orchestrated smear campaign plain and simple.