As fate would have it, one can actually be allergic to Caffeine. There are a lot of symptoms for Caffeine allergies, most of which closely resemble mental disorders (Schizophrenia, Paranoia and Manic-Depression taking the cake).
You might be thinking, "Well, that seems far-fetched." And you'd be right. It does seem far-fetched, but then again so do paradoxical reactions. Caffeine is by far the most available mass-market stimulant, and for most it is exceedingly effective, blocking your ATP receptors, thus not allowing you to realize that your body is drained (in essence giving you "energy"). For a few unlucky individuals, it makes them exhausted, and then there are those poor souls stuck with an allergy.
Imagine, if you will, your morning cup of coffee with a demon sitting in the cup, laying in wait down at the bottom of the delicious nectar of life. You look at your watch. Oh! You're going to be late! You down the last mouthful of coffee and swallow that demon right up. At work you are as productive as always (after checking farmville, of course). Something's different today, though. John, in the cubicle across from you, keeps giving you this look. Like he knows something you don't. You stare hard at him. John smiles and shrugs, before ducking back to his computer screen.
The boss walks by and talks to you about how valuable your work is to the company. You stare at him in wild-eyed terror, then you get back to work. In the reflection of your monitor, you see him talking to John. John laughs. They're plotting. Maybe the company is downsizing? That must be it. You're going to be fired; you're sure of it. No, that's crazy talk. You've been a loyal employee for several years now. Sure there was that rough bit, when you applied to see if you could get another job, but - wait. What if Boss found out? Well, he can't fire you for looking. Besides, the workplace morale went up, and you didn't even get another offer anyway.
You spend the rest of the day sipping coffee and working steadily on your TPS reports. John's glances become increasingly conspiratorial. Eventually, you just kick off work and go back home. Wife is home early. Maybe your boss called her? She looks concerned. You ignore her and eat some dinner quietly. If you're quiet enough maybe she'll slip and say why she's concerned.
While getting ready for bed, you catch your reflection looking at you funny. You duck beneath the counter and brush your teeth. But the mice might eat you if you stay too still. You do something like a crazed bottle dancer from Fiddler on the Roof, and you somehow manage not to choke to death on your toothbrush.
Now imagine that your day went like that EVERY TIME you drank coffee. Especially on days after a long night out.
Yep, Fate's a cruel harlot.