During a college lecture I had what my friend described as a short seizure, for me I definitely felt like I was passing out and I had visions. I carefully walked to the campus hospital and the doctor I met with put forth his best guess that my excessive caffeine consumption had irritated the Vagus nerve causing me to pass out. So that's something to be wary of, oh and the withdrawal headache you'll get on Sunday if you forget you have a pretty hard weekday coffee habit.