Trilogy (3) - July 28, 1978 - S5-E10
Corrected entry: Sam, and not the man he leaps into, somehow fathers a daughter with Abigail. Al and Ziggy's answer to "How?" is "We just don't know." Since Sam has always leaped into the physical body of his subjects with no trace of his own physical presence (yes, we see Sam, but no one else does), fathering a child with his own DNA should be impossible. And the characters acknowledging the "mystery" does not excuse the breach of previously established series canon: it merely makes this, at the very least, a deliberate mistake.
Correction: It is explained several times within the series, particularly in 8 1/2 Months, that Sam himself is physically making the leaps but that he occupies the aura of the leapee - this is why others do not see him as Sam. This is also shown to be the case when he leaps into a blind man but can still see or a man with both legs amputated and yet Sam can walk. This being the case, it doesn't seem impossible for him to have fathered a child during a leap and does not go against canon in suggesting he did.