Result of cross-pollination between vine crops

Growing Cucurbita: pumpkin, gourd, squash
Each year, I buy new seeds to plant pumpkins. I plant only a small area since I just want a few pumpkins to use for fall decoration. Buying a pack of seeds to grow my own pumpkins is a lot cheaper than buying pumpkins grown by someone else.
Another reason why I buy seeds
I prefer buying new seeds from a garden center or catalog because of concern of cross pollination affecting seeds from pumpkins that I grow. Cross pollination can occur between pumpkins, gourds, and squash because they belong to the same botanical species called Cucurbita. In addition to pumpkins, my garden also has zucchini, a summer squash. So there is a possibility of cross pollination in my garden.
How cross pollination of Cucurbita occurs
When pollen from the male bloom of one Cucurbita plant (like a gourd bloom) is delivered to the female bloom of a different Cucurbita plant (like a pumpkin bloom), that is called cross pollination. That cross pollination will not affect the appearance or taste of the current season's crop. The seeds from a pumpkin, squash, or gourd whose bloom was cross pollinated, however, will generate fruit with uncommon characteristics. If you want to use the seeds from your Cucurbita vine crops, you need to prevent cross pollination. Cornell University's plant breeding guidance for Cucurbita Seed Production recommends planting different species one-half to one mile apart to avoid cross pollination.
Back to my garden

That brings me to this year's pumpkins in my garden. I purchased a seed packet and planted about half the seeds. When the first fruit showed, it was yellow. Pumpkins start green and turn orange. Pumpkins do not start yellow. Some of the fruit was smooth skinned (no grooves like you see in a pumpkin) and some of the fruit had bumps.
I contacted the seed producer, sending them pictures of my "pumpkins," and they agreed that the pumpkins appeared to be a result of cross pollination. They sent me a refund of my purchase price for the seeds. I am guessing that pollen from the male bloom of either a gourd or bumpy squash reached the female blossom of the grower's pumpkin vines. What grew in my garden is a result of cross pollination.
Pumpkin that was cross pollinated with either a gourd or squash
Though they do not look exactly like pumpkins, the resulting fruit of the cross pollination event are still festive looking.
Difference between male and female pumpkin blooms

Male pumpkin bloom
Pumpkins are usually pollinated by insects. A pumpkin vine will produce both male and female blooms. I have noticed with my pumpkin vines that there are more male blossoms than female blossoms.
Female pumpkin bloom
You can identify a female bloom by the bulge of an immature fruit at the base of the bloom as shown on the bloom image to the left.