I'm rebuilding a bike (I keep asking questions about it) and find I have the perfect amount of knowledge to be amazingly dangerous to my bike.
My latest problem is the chain. I put a seven speed chain on without realizing. It is cut to have the same number of links as the old chain, and seems OK. However, I was told today (scolded) that the pitch is different and that shifting and wear will both be improved if I put the 8 speed chain on.