I was working on a ship design. I wanted to move an extension pylon and a shield attached to it, to a different place on the ship ( so, purely an aesthetic change) so I first moved the shield to another hardpoint located on a wing piece. When I went to click the pylon to make it active so I could move that, nothing happened (the highlight was still on the shield) so I assumed that I had mis-clicked (it's a small pylon) so I tried clicking the filter to show only extras not functionals. That did not work, and after clicking around madly on the interface I realised that the game had frozen up (had to Ctrl-Alt-Del and close it down manually) and lost I don't know how many turns of progress

I've performed this sort of redesign many a time and this is the first time I've had a problem.