Is it possible that a VOR/OBI can give wrong reading?
The VOR transmitter has inbuilt systems to check its accuracy. If it drifts by more than a degree or two (can't remember the exact figure) it shuts down.
Not always. Depends how well maintained the VOR is! This sort of failure very nearly caused the loss of a BMed A320 at Addis Ababa a couple of years ago. But then in UK airspace its more likely something aeroplane related.