Here's one important example:
The destruction of the Oswald NoteIn 1975, the allegation surfaced that the FBI had destroyed a note delivered to it by Lee Harvey Oswald, just one or two weeks prior to the assassination of President Kennedy. An internal FBI investigation failed to find any records relating to this, but interviews of Dallas Field Office personnel established that an Oswald visit and note dropoff had occurred.
The House Judiciary Committee heard testimony from several relevant witnesses, as did the contemporaneous Church Committee. The results of this were:
* Oswald definitely did visit the Dallas Field Office a week to two weeks prior to the assassination, looking for Agent Hosty, who had recently visited his wife Marina.
* When told that Hosty was not in, Oswald left a note in an envelope which was unsealed.
* The note contained some sort of threat, but accounts varied widely as to whether Oswald threatened to "blow up the FBI" or merely "report this to higher authorities."
* Within hours after Oswald's murder on 24 Nov 1963, Hosty destroyed the note and a memorandum which Special-Agent-in-Charge Gordon Shanklin had ordered written on November 22.
Hosty maintained that Shanklin, the head of hte Dallas Field Office, had ordered him to destroy the note. Shanklin denied ever having heard of the note until 1975, though Assistant Director William Sullivan did recall the incident. The House Select Committee on Assassinations reviewed the incident and did not find Shanklin's denial credible.
The movie JFK added a new twist based on rumors which have never been substantiated. In the film, New Orleans DA Jim Garrison wonders why the FBI would destroy a note which would tend to confirm Oswald's violent character, and presents his staff with an alternative: "This is just speculation, people, but what if the note was describing the assassination attempt on JFK?"
SOURCE:
http://www.maryferrell.org/wiki/index.php/Destruction_of_the_Oswald_Note The note was destroyed minutes after Oswald was killed while in police custody.
If I were advising Bobby Kennedy, I'd have told him to hire others rather than count on Hoover's FBI.