A creamy tuna surprise occurs when you go down to eat out a chick and you abruptly discover, after removing her panties, that you are about to dine on the sloppy seconds from a dude that just dropped a load in her snatch a few minutes prior.
Chet: Yo braaaa! Where ya been?
Devin: Dude I just took that nasty ho out to the car to eat her snatch.
Chet: Cool braaa! How was it?
Devin: Dude, I didn't eat that box there was a Creamy Tuna Surprise waiting for me. What a nasty ho!