From The Cuckoo's Egg by Clifford Stoll:
Someone at NSA had leaked word my research to the Department of Energy. In turn, they were pissed that they hadn't heard earlier--and more directly.
Roy Kerth stopped me in the hallway. "DOE is going to reprimand us for not telling them about this incident."
"But we did tell them," I objected. "More than two months ago."
"Sure. It's in my logbook."
Roy wanted to see it, so we walked over to my Macintosh and brought up the logbook. Sure enough, on November 12th, my logbook said that I'd informed DOE. I'd written a summary of our conversation and even included a phone number. DOE couldn't compalin--we could prove that we'd informed them.
Saved by my logbook.