He was simply trying to bleach the hair around his batcave for a movie role -- when Sacha Baron Cohen reportedly broke out in a rash so bad, he couldn't sit for days.

Several British reports claim Cohen, who was prepping to play "Bruno" in his upcoming Borat-style flick, was taken to a hospital after experiencing what sources are saying was a "burning sensation that spread."

Those who have ever hit Taco Bell at 3 in the morning know the feeling...