The scene: a leafy street in Clapham. Mitcham's partner, Neil Dawson, the executive head of planning at TBWA/London, was still seething, having had his car nicked the previous day. To calm him down, Mitcham offered to give him a lift into town in her silver Cherokee Jeep.
As he tossed his gym kit into the boot, Dawson's jaw dropped below sea level. Expecting only to see Mitcham's green wellies, he was instead confronted by hundreds of DVDs such as Erotic Adventures of Bethany Showers and Petra Gets Pounded in Prague.
There then followed a heated exchange as Mitcham tried to convince Dawson that she hadn't been augmenting her modest Golden Square stipend by moonlighting as a porn queen.
Once the dust had settled, the couple realised what must have happened.
Mitcham regularly parks her Jeep in a Soho car park. It seems that the proprietor of a nearby erotic emporium had sent one of his lads to dump some stock into the back of his vehicle, which also happened to be a silver Jeep. The car park attendant must have innocently handed over the wrong set of Jeep keys.
Happily, the hoard of erotica has since been reunited with its owner.
Which is handy, because you can imagine his wrath had he driven off with the wrong cargo - it's hard to see what a Soho pornographer would have done with a couple of M&C brochures and a pair of Jimmy Choo shoes.
An audibly shaken Dawson told Campaign: "It was awful. I've never seen such filth. It was enough to make a grown man weep.
"I had to watch every DVD twice just to be absolutely sure how shocking they were."
"He means three times," Mitcham sighed.