Don't feed dogs human food, that's one big no no. Also doughnuts aren't really healthy for people, so think about what could happen to your dog. Deep fried pieces of dough yum...
Not dangerous, But not healthy either. One doughnut is not going to hurt a mid sized dog if eaten by mistake. Don't make it a habbit. Too much sugar and grease. Probably will get the runs for a while from it.