That is not going to happen. I have phone records, emails, my own personal records. The very notion that someone recorded me saying something very specific for several seconds saying a bad thing without providing the greater conversation raises questions. Why didn't the person provide the entire conversation? I don't work off camera, most people don't. Your bosses work on the same theory I do, record everything, so most places are recorded, so it happening off camera is unlikely. This is my workplace, because it is mining, but I also have to tag into my office with an access card, every worker does. To open any door you tag, even the break room. Why didn't my tag register to that specific room? Why didn't my office register me leaving by me closing the door? It doesn't work in my situation, and for most workplaces.
For that person to lie successfully they need to account for so many variables. Which is why I say record everything, because you don't end up having to cover for variables.
Lying casually without scrutiny is easy. Nobody usually bothers to check so people get away with it. Lying with scutiny, that is stressful and impossible. A good lolcow example right now is Nick Rekieta. The guy is a habitual liar, has been all the way along. But one of the things that has really spiralled him, is the scrutiny. He is having to maintain, modify and keep track of all the lies, whilst being an addict, it isn't the one and done deal most people are used to, and that pressure is breaking Nick. It means changing things and playing catch up as new details emerge, to keep the lie a person ends up telling more lies, which means more effort, more things to keep track of, more stress. It is nearly impossible to successfully maintain a lie under scrutiny, everyone cracks without help or support (and that involves paying lawyers a lot of money). By recording everything a person doesn't have to lie, they don't have the stress of having to recall, they simply pull out the notes and they've got reference. Simply put a lie of the type you've proposed takes time and effort, that is only going to get worse for the liar as good record keeping means a person can successfully and truthfully account for all of their details with very little effort. The risk vs reward is not worth it for the liar, and it becomes not worth it for HR as they now weigh the cost of an unfair dismissal suit with an unreliable narrator, even if they win it is going to cost.