This came into my mind recently, after I attempted to list the possible formations of the verb "stutter".
You can't say "be stuttered", since you can't apply the verb "stutter" to someone else, or something like that, but you can say "be stuttering", since it means to exist in that state.
I can't think of other verbs that could be like this, though, so I wondered, is there a general rule regarding this?