Luke 24:37

‘But they were terrified and frightened, and supposed that they saw a ghost.'

But Jesus' sudden appearance among them unnerved them. The problem was that this was not an hallucination, it was real. Notice the multiplication of words, ‘they were terrified and frightened', for they genuinely thought that Jesus was a ghost. How else could He have suddenly appeared among them like this? (They had necessarily previously had no experience of things like this, so their fears were understandable).