When performing a poomsae, you should finish on the same spot where you started. Poomsaes are designed to be symmetric, and this should happen naturally if all the steps are done correctly and with exactly the same length.
That is the theory. But for the Taegeuk Il Jang, practiced by 8th Gup grades, this seems harder than it should, and you will usually end significantly behind the starting position.
The Taekwondo Wikia describes this problem, and even adds a graphical explanation, so it seems to be a known problem:
Assuming one performs the poomsae with consistent stance lengths, by the time one finishes the form, one is somewhat behind the starting spot. Specifically, if all your stances are "perfect" you will finish 4.5 apseogi-widths behind the starting spot. The problem is exacerbated if your apseogi-widths are too wide, which is a common mistake.
The solution is perform the sideways ap seogui with the feet close to an imaginary line, not too spread apart, and to compensate the error by performing the two final ap kubi steps, going back to the starting position, a bit shorter than usual.
Is this some kind of "design flaw" of the pattern, or is this just an execution flaw, in the the steps and turns are not being properly done?