In the introduction, FitzGerald explains that "The original Rubáiyát... are independent Stanzas, consisting each of four Lines of equal, though varied, Prosody, sometimes all rhyming, but oftener (as here attempted) the third line suspending the Cadence by which the last atones with the former Two." Basically, the usual rhyme scheme is AABA.
In the first edition (which is what my copy follows), stanza LXIII, located amongst a string of stanzas about clay pots, reads:
None answer'd this; but after Silence spakeA Vessel of a more ungainly Make:"They sneer at me for leaning all awry;What! did the Hand then of the Potter shake?"
The AABA rhyme scheme emphasizes the incongruity of that "awry." The other lines all rhyme with each other ("spake," "Make," and "shake"), but the third line stands out in the same way that this particular pot is ostracized because of its form.