One misunderstanding I see quite often is relay types. With most input/output nowadays being solid state such as an open collector, or isolated optical, the use of relays have become a lot more limited than in the past. Yet many manufacturers still label their equipment to be synonymous with a type of relay.
Form A relays contain SPST (Single Pole Single Throw) contacts that are normally open. (The relay is open in a non energized state)
Form B relays contain SPST (Single Pole Single Throw) contacts that are normally closed. (The relay is closed in a non energized state)
Form C relays contain SPDT (single Pole Double Throw) contacts. One set in normally open and one set is normally closed, and these two sets change state when energized. A DPDT (Double Pole Double Throw) relay is essentially two SPDT contacts that are energized from the same coil.