It's common sense, but you'd be surprised how many people don't heed that advice. I've attended some memorable office parties over the years, and seen some things that I couldn't believe I was seeing or hearing. These are all true stories, and they're from multiple different companies.
*One year somebody thought it would be hilarious to "spice" up the gag reel of that year's bloopers with clips of gay porn.
*Another year I saw a bunch of co-workers congregating in the DJ booth, so I walked in to request a song (like I assumed they were doing), saw little white lines on the counter, and was told: 'Rick, you didn't see anything.'
*One year a co-worker and a boss (one of each gender) disappeared into the unisex bathroom (at the same time). Their spouses were both at the party.
*One year an ex-employee showed up to apologize for the way he left the company, and a fist-fight nearly broke out between him and the boss. They had to be physically seperated.
*That same year a huge drapery caught on fire and had to be put out with a fire extinguisher.
*I arrived at one party a little late, only to discover it had set sail and was out on Lake Michigan.
*One year a boss blatantly and graphically hit on my wife...right in front of me.
*We were seated at a table with co-workers I barely knew one year, and the woman next to me spent the entire night ripping her ex-husband, who I had never met.
*One year the boss banned Christmas music because he personally hated it. "You guys can do your whole Jesus thing somewhere else."
My old Loop colleague Buzz Kilman used to tell me that there were only a few hard and fast rules in the world, but one of them was "always go to your office holiday party". I used to think he was right, but as I was writing the examples above, I started to reassess the wisdom of that advice.