As Bernard (kid with glasses), you will eventually pick up the Regular and Decaf coffee pots. These are located in the "Break" room.
From the Main Lobby where you start the game, open/go through the double doors underneath the WELCOME banner. Then, go through the Swinging door (no need to open, just click on it) located just to the left of the fireplace.
Once in the "Break" room, both pots will be located on the coffee warmers on the left of your screen. They are labeled R for Regular and D for Decaf. Once you have picked up both of these items, open your inventory, click on either pot, then on the other. You should see the option "Use coffee with decaf coffee" or the other way around, depending on which pot you choose first.
Bernard will say "Never mix, never worry!" while your achievement pops.
You can also get "Voiding the Warranty" in this room by picking up the Fork and attempting to put it in the microwave. See my solution to that achievement for more details.
If you see a way to improve this guide or want further clarification, leave a comment! Thanks!