Wheat bread's about 75 calories a slice, so 150 for just the bread. I don't know about you, but I put it on thick—maybe a serving-and-a-half per slice, which according to the Nutella website is 570 calories. So 720 total. Not 1,000, but closer to it than 239.
A serving size is 2Tbsp, which is what I put on a serving size of bread (1 slice) when I'm paying attention to how much I'm putting on. Like I said, though, I tend to slather it on (mainly because Nutella is so damn good)—so I'm really getting 1.5-2 servings of Nutella per slice.
Two slices—75kcal * 2—and a hunk of Nutella—190kcal * 3—is way the hell more calories than I should be eating as a snack (720kcal). Veeeery easy to go overboard.