Yeah, the most common reason for failure. However, only c. 1% of those who go for a Class One initial do fail, so you should be ok
As far as I can tell, if you're fit and healthy, as long as your eyesight is ok, you are unlikely to find a previously undiagnosed condition! I got very nervous before my intial (2 weeks ago) and asked the same questions, but it was all fine; the staff in the Aeromedical department at Gatwick are really nice and put you completely at ease!
Good luck; you'll be fine!