A few years back, a young friend of mine took a job at Starbuck's while going through college. He and his co-workers (also college mates) devised a wicked drink consisting of 4 espresso shots mixed with Red Bull to help them stay awake to study for several nights before finals.
He ended up in the hospital from caffeine overload. While
his intake was indeed extreme, it definitely convinced me that one could overdose on caffeine.