Esspresso - Comforting goodness or lethal killer?

Have you ever had too much caffeine and got a little shaky and felt your heart beating faster--like it was going to jump right out of your chest? After watching a co-worker order a latte with 4 shots of esspresso and hearing that he had a depth-charge (3 shots of esspresso with coffee) earlier in the day I was bound to consider this...Is there a lethal amount of caffeine?

We asked out barrist, "Do you have a limit to the number of shots you will serve someone"? (Thinking that this must be similar in someway to a bartender cutting off a drunk patron) Our answer was "No, there is no limit". So, naturally we followed up with, "Well, what was the most number of shots you have served to someone"? The answer was "10". Ten!!

How many shots would it take for your heart to explode? Discuss.

Extra credit: What occupation was the customer who ordered 10 shots?