I bought my cat a new scratching post (a basic one with a square base, central pole and string and ball on the top) and was alarmed to see her tip it over the first time she used it! I compared it to the old tatty one (of a similar design) that I was throwing out and realised that the base was significantly smaller on the new one (presumably to reduce manufacturing costs). The next time she played with it she was holding the base down with one paw (to stop the thing toppling over again) and clawing at the post with the other paw! So, I've now had to get a large square piece of wood and attach it to the base to make it more stable so she can use it as intended. With this modification it's working fine.