This prank will both annoy and confuse your victim, causing them to question their very sanity (if they're paranoid enough). With a bit of luck, or an acquired skill at misdirection, you can pull it off!
After you follow these simple steps, the victim will wind up unsuspectingly drinking Tabasco or another brand of hot sauce from their beverage, but no one else will be able to taste it, leaving your victim both looking and tasting off-kilter.
Warning! Don't try this on anyone with any sort of medical situation which might result in them having any sort of attack; I'm not responsible for your actions.
Step 1: Situation and Materials
This prank require a specific set-up. Your victim must be drinking something from any kind of container through a removable straw. The only thing that you'll need to supply is the hot sauce, and maybe a distraction, although this can be done if the person is either already distracted (i.e., in a heated discussion or trying to pick up a girl) or if they have excused themselves to go to the restroom. One more suggestion for preparation, please try to wash your hands sometime before enacting this prank; no need to add insult to injury.
Step 2: Step One
While your (ex-) friend isn't watching, remove the straw from their drink and simply dip it into the hot sauce, covering the top of the straw with a finger before removing it, so that a good amount of hot sauce is in the straw.
Step 3: Step Two
Place it back into the drink and, when the straw is all the way down and a good portion of it is under the liquid, release your finger hold. Since most of the beverages that we drink are water-based, both your victim's beverage of choice and the hot sauce will have similar densities, and so the hot sauce will remain in the straw instead of releasing into the rest of the cup and simply mixing with the beverage.