Just had the experience of DS biting both myself and DD this morning. It happened when he was very excited, we were playing in a home made tent, he just came up to me and chomped down on my arm, then 10 mins later, bit his older sister. Again when he was very excited.
DS is just turned 16 months and has never bitten before, never hit out, and i want to "nip this in the bud" pardon the pun!