These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
And of course, since all we know about him is how he's portrayed in Edward's self-aggrandizing stories, we have no idea how he really was or how the affair played out in real life.
Or if he even existed.
Big Lipped Alligator Moment: At one point, Will sees a large fish swimming in Edward's pool when he goes to clean it, but it quickly vanishes and he never sees it again. The incident is never mentioned after that. We never find out if the fish was really there (or if he just imagined it) or how it wound up in the pool. Or, for that matter, how a fish could survive in water laced with chlorine...
This might dip into WMG territory, but this happens about the time Will started softening up to his dad's tall tales. Maybe he's finally coming around to his dad's perspective?
When Don sees his future death in the Witch's eye, it makes you wonder how it will affect him later. Even Edward admits that seeing your own death "could kinda screw you up", but this is never brought up. Then again... later, Don shows that he's so desperate to marry Sandra that he'll beat Edward to a pulp for intruding on her, even though Sandra obviously doesn't love him and leaves him for Edward at the drop of a hat. Why is that? Could it be that he knew he was going to die soon, and was terrified of dying alone? Tragically, Don's desperation to avoid this ends up leading to his death.
The story about Edward catching the catfish on the day of Will's birth is the very first story that we hear, and it's treated as a sort of Establishing Character Moment for both Edward and Will. Edward keeps telling it up until Will's in his 30s, even whipping it out on Will's prom night and at his wedding, giving the impression that he's self-centered and oblivious to his son's coming of age. We see Will gradually getting more annoyed at the story as he gets older, eventually accusing his dad of being a pompous glory hound who hides behind elaborate fantasies because he can't face the real world. But then at the end, we find out that the catfish story is, in fact, one of the only stories that Edward completely made up—all of his other tall tales were just exaggerated retellings of things that actually happened to him. The story then takes on a completely different meaning when you realize that Edward romanticized the story of his son's birth to make up for not being there to see it. He wasn't telling the story out of ego...he was telling his son how much he loved him in the only way that he knew how.
Regarding the exaggeration in the stories, how much is from the way Edward tells it and how much is from Will's imagination? For instance, if Edward mentioned the Siamese twins, Will could've pictured conjoined twins when really they were just twins from Siam. Or in regards to Karl, pretty much anyone would call an almost eight-foot-tall person a giant, but a child would picture someone called a giant a lot bigger than just eight feet. That doesn't explain the werewolf, though...
The twins are "joined at the hip" in the flashbacks. Edward could have simply used the popular expression, while his son mistakenly thought he was being literal.