Hilarious but spoonerisms often are.
I heard one once that was so bad (given the situation) I almost died laughing.
It was just before the start of the second year at Oxford (I was back early for a resit, another story). Christ Church was one of those colleges that by and large managed to accommodate most undergraduates on site.
Having gone through a lottery process, a lot of guys in the college were sharing suites in the Peckwater quadrangle. Each was typically an old fashioned study with separate bedrooms. Two physicist friends happened to be sharing, one the son of some fairly traditional Iranian parents, the other a regular boyo from the valleys, both sound guys

Anyway, parents of said Iranian chap were visiting and were keen to meet his friends so a whole bunch of us had turned up to say hi. His dad commented on the structure of the suite and asked how they managed to study if either of them wanted to relax or have friends round instead. So, the welsh guy replied, "Oh, it's OK, we have separate desks in our rooms"
Or at least, he thought he did. What he actually said was:
"Oh, it's OK, we have desperate sex in our rooms."
Well, there was a moment of silence that felt like an eternity as I fought the muscles in my face and tried to quell my shoulder heaving... Then the entire room just fell about laughing :lol: