A fruit is actually the sweet, ripened ovary or ovaries of a seed-bearing plant. A vegetable, in contrast, is an herbaceous plant cultivated for an edible part (seeds, roots, stems, leaves, bulbs, tubers, or nonsweet fruits). So, to be really nitpicky, a fruit could be a vegetable, but a vegetable could not be a fruit.
an interesting list of fruits that are often thought to be vegetables:
tomatoes
cucumbers
squashes and zucchini
avocados
green, red, and yellow peppers
peapods
pumpkins
I copied the above from another resource.