Mostly I've seen odd results trying to replant seed from cucumbers, squash, pumpkins, that variety of thing, when they are hybrid varieties. A lot of times, there's a clear regression back to the base varieties that were hybridized, and it can be quite a striking difference.

Also, a number of these cultivars can cross-pollinate, and then you can get some really interesting monstrosities. For instance pumpkins getting crossed up with buttercup squash that come out as dark green and orange tiger-striped things.