Thanks for the advice everyone, its much appreciated.
I got the caffeine pills from Amazon. They contain 200mg caff each and on the special occasion I swallowed 4 all at once, so I'm sure I had 800mg. Maybe I was very lucky not to require medical attention, especially considering I did the ALS ice bucket challenge when about 400mg was in my system (presuming the half life is about 5 hours).
Anyway, I won't be taking this much again since clearly it is a bad idea, even on rare occasions. I'll stick to 1 or even half a pill.