The FBI-issued cell phone of Peter Strzok, whose previous texts to his mistress (also an FBI employee) showed fierce hostility to Trump, suddenly had problems due to "software upgrades" and other issues — and voila — all the messages between the two from Dec. 14, 2016, to May 17, 2017 vanished. Strzok, who oversaw the Trump investigation from its start in July 2016, was removed from Mueller's Special Counsel investigation last summer after the Justice Department Inspector General discovered his anti-Trump texts.
Stephen Boyd, the assistant attorney general for legislative affairs, notified a Senate committee that "data that should have been automatically collected and retained for long-term storage and retrieval was not collected." The missing texts could have obliterated the remnants of credibility of the FBI's investigation of the Trump campaign.
Conservatives are caterwauling about the vanished evidence but this type of tactic has long been standard procedure for the FBI. Acting FBI chief Patrick Gray was forced to resign in 1973 after it was revealed that he had burned incriminating evidence from the White House in his fireplace shortly after the Watergate break-in by Nixon White House "plumbers." Gray claimed he was resigning to preserve the "reputation and integrity" of the FBI — but that hasn't worked out so well.