POA Ch-5 - He forgot that he remembered.

[Steve]

No: HPFGUIDX 181612

I found an interesting quote in Chapter 5 of POA. Harry has
just come off the train, and he and the gang are about to 
board the coaches that carry them to the castle -

---POA Ch5 pg 87 Am Ed HB ---

Harry, Ron, and Hermione followed the rest of the school along 
the platform and out into the rough mud track, where at
least a hundred stagecoaches awaited the remaining students,
each pulled, Harry could only assume, by an INVISIBLE HORSE,
because when they climbed inside and shit the door, the
coach set off all by itself, bumping and swaying in procession.

--- end quote ---
(emphasis is mine)

Remember this is really the first time Harry as seen the coaches.
The previous year (CoS), Harry and Ron arrived by flying car. So,
Harry first sees the coaches and assumes some invisible horse is
pulling them. But that thought quickly fades and he later assumes
they are just propelling themselves along. Until after GoF when
he first see the Thestrals. 

This is one of those odd occasions when Harry knows but doesn't
realize he knows. Instead he picks up on the false assumption
made by everyone else and accepts it. 

I just thought it was interesting that so early on in the series
Harry rightly assume some invisible horse(like) creature is
pulling the coaches, only to forget that he ever thought of 
this idea.

Steve/bboyminn