Triffid?

Miss Flum was minder for the cat next door at holiday time and on one occasion was rewarded with a small cactus in thanks for her efforts. It was the size of one small stem (a third of the thumbnail) in a 2" pot.

It grew slowly and steadily until it filled the little pot so Miss Flum, considering it to be overcrowded, repotted it into that in the photo. In a matter of months it has developed into the magnificent rampant specimen you see here. One might be rather proud of it but for the fact that it is covered with sharp spines and is growing so close to the hand wash (removed for the photo) that it creates a scratch hazard.

I am hoping that she finds it a position in her own house before it takes over the windowledge.