“But a man must examine himself, and in so doing he is to eat of the bread and drink of the cup. For he who eats and drinks, eats and drinks judgment to himself if he does not judge the body rightly. For this reason many among you are weak and sick, and a number sleep.” I Corinthians 11:28-30
When I was an evangelical I always found it interesting that the pastor would go out of his way to emphasize that the cups are filled with simple grape juice and nothing more. Then he would cite the above verse and caution us to avoid taking communion “unworthily”. That seems like fairly severe consequences for misusing something that is merely grape juice.