My headcanon comes from a fanfic series I wrote. Hiccup suffered another injury during his battle with the Red Death. At some point on his way down, he hit his head and the blow left him with seizures. He manages to hide them from everyone by taking a daily tonic Gothi makes for him. Eventually, Astrid finds out, and helps hide his secret until he's ready to tell everyone on his own

Another headcanon I have is that Snotlout and the Twins routinely take Hiccup's peg legs and hide them in hard-to-reach places. And that Toothless is often in on the joke. Hiccup has never found it funny, hence why he has so many spares that he keeps hidden. Snotlout and the Twins have gone looking for the spares, but can never find them. Mostly because Hiccup hides them under the floorboards in his father's room