Coworker threatens to report colleague to HR after a friend brings them coffee at work: 'They received an email from an unnamed employee who felt I might have received more than a coffee'

Why would you call HR over something as small as a cup of coffee?

There are lots of ways coworkers can annoy each other. After all, they spend almost all their time with each other. Some issues are bound to come up. Maybe you have a coworker who types really loudly, and it's super distracting to you. Maybe someone has a bad habit of playing music out loud, and you can't focus around them. These are relatively small issues, but the scale of possible coworker annoyance goes all the way up to 11. The ultimate form of transgression in an office is threatening to report someone to HR. This is the final buck of responsibility when it comes to interpersonal conflicts with coworkers. This is where direct action can be taken against you, if they feel so inclined. But it's why the department exists in the first place. 

But sometimes communication breaks down somewhere along the way, and you end up in a passive-aggressive battle with someone you're supposed to be working with. Maybe one of you isn't willing to communicate the old-fashioned way, so they take back roads and alternate channels to annoy you. This has a way of taking over your working life and consuming the way you feel at your workplace. Nobody likes to show up to their office and feel like someone has it out for them. You want to feel welcomed and reassured at your workplace, but some pesky coworkers can take that away from you.