As reported in the April 17, 2002 issue of The Lancet:
A 44-year-old man presented in May, 2001, with muscle cramps. He had no medical history of note, but volunteered the fact that he had been drinking up to 4 L of black tea per day over the past 25 years. His preferred brand was GoldTeefix (Tekanne, Salzburg, Austria). Since this type of tea had given him occasional gastric pain, he changed to Earl Grey (Twinings & Company, London, UK), which he thought would be less harmful to his stomach. 1 week after the change, he noticed repeated muscle cramps for some seconds in his right foot. The longer he drank Earl Grey tea, the more intense the muscle cramps became.
After 3 weeks, they also occurred in the left foot. After 5 weeks, muscle cramps had spread towards the hands and the right calf. Occasionally, he observed fasciculations of the right adductor pollicis and gastrocnemius. Additionally, he noted distal paraesthesias in all limbs, and a feeling of pressure in his eyes, associated with blurred vision, particularly in darkness...
The patient assumed that there was a relation between his symptoms and his tea consumption, and stopped drinking Earl Grey after 5 months, reverting to pure black tea again. Within 1 week, his symptoms had completely disappeared. Symptoms also remained absent if he completely withdrew from tea, which he did in the nature of experiment, for about a week. He found that his symptoms did not recur as long as he consumed no more than 1 L of Earl Grey daily.
When last seen in November, 2001, neurological examination, nerve conduction studies, and electromyography were normal. He was still drinking 2 L of plain black tea daily (his entire fluid intake), and had no complaints.
The moral of his story is that 2 liters of tea a day is apparently fine. But 4 liters is asking for trouble.