It is a sofa stuck in the staircase on the way to Richard MacDuff's flat.
The sofa was bought by Richard MacDuff, but during the delivery it got stuck in the staircase halfway up to Richard's flat, and could neither be moved further nor back down and out. Richard even asked the help of a neighbour's spatially smart son, as well as simulating the problem on his computer, but all pronounced the sofa impossibly and irreversibly stuck.
When Richard, Dirk Gently, and Reg returned from visiting Samuel Taylor Coleridge, Reg's flat materialised its door in the wall halfway up the staircase to Richard's flat. They then traveled back in time three weeks for Richard to make a reservation at the restaurant L'Esprit d'Escalier, which coincided with the time the sofa was delivered. While Richard made the phone call, his friends opened the door for the delivery people, who thus were able to manoeuvre the sofa around by utilizing the extra space of the open doorway. The sofa was then impossibly stuck once the door was dematerialised again.