I've seen a couple of cases of cats being unwell after flea treatments - one was given a dog spot on that contained permethrin, and stated that it was toxic to cats, sadly I couldn't save her despite hours of intensive treatment.
The other had broadline, which is fine for most cats, but he had licked some from his fur and was unwell not eating and drinking, but with supportive treatment including a drip, he recovered well and the owners changed the brand of flea/wormer and he was fine thereafter.