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I've had them as well as a kitty that loved chewing on it and it never hurt her. It's also not on the list of plants poisonous to cats. Just a little extra info, if you're thinking of buying one, they are indoor challenged. Click here for more info
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Your original question was, "Are majesty palms poisonous to house cats?", which I answered.
As for the grass, many cats (and dogs) will chew on grass, and it's not harmful to them. It can act as an internal cleanser, sometimes it makes them vomit, sometimes they don't vomit, but it contains a lot of things that are good for them. See this link for more info.