Some types of cancellations hurt your rating less than other types, but Fiverr policy does not clearly mention which ones will hurt more/less. The default message for all cancellations is “It MAY hurt your rankings”. Keep it to a minimum if it is avoidable, but sometimes it’s inevitable.
Let me go out on a limb and say that “buyer ordered this gig by mistake” shouldn’t hurt as much as “the seller is not able to deliver this gig”.
Orders cancelled by CS do not have a negative effect on your account. I just had an order cancelled by the CS at the behest of a buyer who encountered a bug which would not allow him to submit order requirement data. This is what the cancellation email said :