For example, a pair of asterisks. Worse still, even if you're aware of this it can bite you in surprising places. Since the editor considers text in a list item to be at the start of the line, even typing "#" in a list item will blow up.
Apologies for the delay on this. I've spent a lot of time trying to come up with a solution that meets the needs of all users, but we really just need a quick patch to stem the bleeding. The easiest (and perhaps, least surprising) thing to do for the moment is just to escape those characters, which CommonMark supports with a preceding backslash.