a) it goes over like a lead balloon because it wasn't funny to begin with, or
b) the intended audience didn't "get it" or misconstrues the pun as something else so hackles rise, a flame war ensues, testosterone levels soar, war breaks out, nations topple, and the Earth is left a vast, barren, charred cinder with melted Anita Bryant CDs lying all over the place.
"Whoa, doood--you know, racing motorcycles and cars. Racer. Racist. Get it?
"Oh. Well...FY, anyway."
(That is a case of a word malpunction (or just malpunction.))