Just trying to draw some plate washers and this is the second time now that this has suddenly happened: all the lines are suddenly extended (infinitely it would seem). The dimension lines indicate what should be the corners of the pieces, the top three of which which are elongated trapeziums.
You have Construction Layer on at layers 0 and Dims. Switch that off. It's purpose is to help for certain operations.
By the way, you use an outdated version. Very much recommended to update to 2.1.3 or even 2.2.0 rc.
Many thanks. I hadn't involved myself with construction lines so I must have somehow inadvertantly toggled it on!
My version is the one in the Linux Mint repository. I know they often lag behind but I tend to play it safe as I've had no end of PPA problems when I've added them.