Cucurbit hybrid questions

In a comment to last week's Friday Fabulous Squash flower Erin is perplexed."While you're on the topic of cucurbits, I have a question for which I have not been able to find a satisfactory answer. Last year I was given a few poona kheera cucumber seeds. It was a lovely vegetable that I planted right next to some other variety of cucumber. At the end of the season, however, I decided I would save the seeds from one of the poona kheera, despite the fact that it had likely crossed with its neighbor. This year, rather late in the season, I planted those seeds, the only seeds I personally saved from last season. When they sprouted and began to grow, I was very surprised to find that they had watermelon leaves. There were watermelons growing nearby in my garden last year, but I didn't have any worry about any kind of cross between a Cucumis and Citrullus. I know what I planted, though, and I know the difference between a cucumber leaf and a watermelon leaf. Unfortunately, none of the plants have grown much and have not flowered. I would love to seed the fruit it would produce, if it could produce fruit. What in the world happened here, Science? I'm dumbfounded."
The honest answer is that TPP doesn't know what happened. But let's examine what is known. Erin grew poona kheera cucumbers, an Indian variety of which there are several,e.g., dosakkai, sambar, as well as other varieties. Erin correctly understands that pollination of one variety by other varieties is both possible and likely depending upon flowering. Watermelon is a different genus, and as far at TPP knows, this intergeneric cross does not occur. Cucumis melo, all of the other melons, might cross with a cucumber, but that isn't the question. The suspected hybrid seeds had incised leaves (assumption - no picture?) that looked like watermelon leaves rather than cucumber leaves. So there it is, only one observation, watermelony leaves, that anything happened at all, and in no way does TPP doubt the accuracy of the observation. We will also assume that the seed looked like typical cucumber seeds, and nothing like watermelon seed or Erin would have noticed.
The first thing to try is to see if a cucumber fruit, any variety, will develop if pollinated by watermelon pollen.  Bag a female cucumber flower and the morning it opens, pollinate it with pollen from a watermelon. Pipe cleaners work OK as artificial bees. Rebag for a day, and mark the fruit stalk with a tag. See what happens.  5-10 trials should suffice.
Sorry to leave this little mystery unsolved, a rather unsatisfying situation, but that's how science is, no answers without data even though Erin was correct about everything.