That person. The one who absolutely destroys a good moment by taking a joke too far or bringing up a sore topic. This can be due to the person's character, thick, but well meaning trying to hard to fit in, or it can just be attributed to one being in the wrong place at the wrong time.
Jeff: Haha, hey man, that was a funny joke!
James: Yeah, totally!
Jim: I like the part where you say " and THEN!".......... It was great. Just great.
(long, awkward pause)
Jeff: Buzz Killington?
James: Buzz Killington