χ accepted as "almost correct" when it's a very different letter/sound than
I never thought my familiarity with the IPA would get me in trouble.
χ is /x/, the voiceless velar fricative, and
ξ is /ks/, a cluster of the voiceless velar stop and the voiceless alveolar fricative. So whenever I'm prompted with "x" I immediately think /x/, not /ks/.