Behold baby Typhon (or Typhoeus).
There are a few sources that claim that Hera gave birth to Typhon without a union with any man. Others say that Typhon was born by Gaia and that it was Hephaistos that Hera miraculously gave birth to (in both cases she ended up with something that was not quite what she had hoped for). It was a bit hard for me to decide which of the myths to choose, since both had their good points. Typhon is more scary and would definitely freak Hera out a lot more, making it more understandable why she hasn't returned to Olympus. On the other hand, Hephaistos has always felt more like Hera's child than Zeus's. It was only Hera that Hephaistos took revenge on by sending her the chair with invisible fetters. True, it was Hera that once threw him out of Olympus, but did Zeus do anything to find him and bring him back? No. After mother and son had reconciled, it also seems like Hephaistos developed a genuine affection for her. But I can't see that kind of affection between him and Zeus.
Anyway, according to my version Hephaistos is the son of both Zeus and Hera.
I really thought this was going to be about Hephaestus. But of course, there always are other, conflicting myths.