Mistletoe (Phoradendron leucarpum): Swallowing too many leaves or berries can cause nausea and vomiting.
Poinsettia (Euphorbia pulcherrima): A mouthful of the plant will only cause some upset stomach, such as nausea or vomiting. You don’t want to make your holiday table salad out of poinsettia leaves, but a taste amount is not a serious problem.
Holly (Ilex spp): Keeping in line with the other holiday plants above, this is another one that can cause nausea and vomiting if swallowed.