The FBI rarely records interviews and instead relies on written summaries (known as Form 302s) which "are widely held up in court as credible evidence of conversations," the New York Times noted last year. Though defense attorneys routinely debunk the accuracy and credibility of 302s, prosecutors continue touting FBI interview summaries as the voice of God.
This is not about FBI agents testifying to something they saw or heard in sudden and unpredicted situations ("I was exiting the bar when I saw two men in ski masks and carrying shotguns enter the store across the street").
This is about FBI agents testifying on the content of pre-planned, formal interviews which can and should be recorded on audio or video, but which they instead take necessarily subjective, even if not intentionally dishonest, written notes on.
When a law enforcement agent intentionally avoids making an objective and complete record of an interview, there's a reason -- and his subjective and incomplete notes on that interview, or his testimony from memory, should be held inadmissible as evidence. It's like being able to take fingerprints at a crime scene, but instead just testifying that a handprint looked to be about the same size as the suspect's and expecting that to be treated as credible evidence.
"Testilying" is a thing. It's always going to be a problem. The courts shouldn't actively facilitate the practice by allowing bullshit like this.