A goof seen in archive footage from City of... (1999) that appears in the opening credits of every episode is when Angel is walking down an alley. In a puddle of water you can see his reflection. Though originally a mistake, they choose to take it through the whole show, as a symbol of his soul.
For a large portion of season 5 Spike is incorporeal. He is not meant to be able to interact with physical objects, and is seen passing through people and walls. However as is the case in most scenarios of this nature, he is inexplicably able to sit in chairs or occasionally interact with larger set pieces.