Then this plant started flowering and setting seed pods and my more experienced readers confirmed that not only was this not a cantaloupe plant, it was most likely a weed of some sort. Lou Murray, Mr. H. and Anonymous commented here and here that it looked like something in the Indian Mallow family.
Well, I let the plant live out its life and I'm now positive that what I accidentally grew was Velvetleaf in the Mallow family. I found a few websites that provided lots of information about Velvetleaf.
The scariest piece of information that I learned about Velvetleaf, which is actually a pretty seriously invasive weed, is that its seeds can remain viable in soil for more than 50 years. FIFTY YEARS!
And I grew it in a five-gallon bucket. With a stake for support.