Blunder of the Season (I hope)

First round of the Dorset Closed Championship and I'm feeling reasonably happy with this position, which came from a rather poorly played line of the Queens Gambit Accepted.

How often is it unsafe to complete development by castling after an opponent has just made a K move? Almost everytime, yes?

Black has just played 14...Kg7, and without thinking I returned to the board (metaphorically) and swapped my K & R. As I was writing the move down it occurred to me that Rc8 would be a nuisance. Wrong again.

Not a nuisance but a disaster. Black's rook was sitting on c8 before I had even looked back at the board. Disgusted with myself, I resigned after sticking my Q on a6. But to make it worse, my opponent - whose 161 grade presumably means he's not always so wrong about things - started droning on about how the position was obviously loads better for him even before my wretched castling ("of course the a-pawn is going to just run down the board").

I asked black to take a look at it with me and show me just how he was going to "run the a-pawn", which he did for only a short while (black's pieces are a bit awkwardly placed, really).

Gah!