Bests & Worsts Reviews from Amazon

The first couple of days after applying this product to my cats it seemed to be working great. Especially for my cat who seems to be very sensitive to fleas. However today I watched one of them fall off a fairly tall piece of furniture twice (something he is on every day and has never fallen off of before) and strangely like he went limp or something. Then a little while later I watched as my other cat who was just standing on the floor fell with her back legs slamming together and her front paws trying to get a grip. Then I read on a few different websites that it has been found that in RARE occasions this product can have neurological effects. I then found two people on yahoo answers claiming this same phenomenon happened (back legs going out) when they gave their dogs advantage. If this is so rare.. what exactly is the probability that it would happen to both of my cats? I won't risk using this product again.