The "officially" voiceless alveolar-palatine affricate does not exist in English. But I can clearly hear it in the sentence "Ouch that hurt" (when the computer reads this sentence and often when actors say it in the movies). I am Polish and for me the English "ouch" sounds the same as the Polish "auć" (and means exactly the same). I am curious about your opinion on this.
1I'm pretty sure that for me ouch has the normal postalveolar affricate [tʃ]. I believe Polish people usually hear as nearer [tʂ] than [tɕ]. It's possible that in this case, the existence of a near-homophonous Polish word, with the same sense was enough to push your recognition of the same [tʃ] sound (which is intermediate between Polish's <ć> and <č>) in the other direction– TristanOct 13, 2020 at 14:47
Somewhat NSFW, but I've been pondering on whether people can tell the difference between the pronunciation of Polish word być [bɨt͡ɕ] and bitch in English (/bɪtʃ/).– FizzNov 30, 2021 at 8:00
FWIW, I think that the dipthong in my American "ouch" is lower and less rounded than in RP, and under the influence of the [ʊ̞] (?) my "ouch" is closer to [aʊ̞̯ʈʂ], with a retroflex t, while "itch" is probably [ɨtɕ] (with the [ɕ] closer to Japanese [sɯ̟ɕɨ] than American [suːʃi]).– David MolesDec 6, 2021 at 20:58
As a speaker of American English, I don't find any difference in the final consonant of ouch, pouch, slouch, poach, reach and so on. I might imagine someone pronouncing "ouch" abnormally for paralinguistic purposes. Using Esling's demonstration table, it doesn't strike me that the ɕ token is like English, whereas ʃ is. I'm not voting on Polish: however, the categorization of segments in the IPA is not based on Polish, or English, it's based on an independent standard. <siV> in Polish may well be closest to IPA ɕ, but it is not necessarily exactly the same. What you would really want is a panel of experts to classify various sounds, so that you can uncover the range of variation that is consistent with the letter ɕ (which English ʃ would not be).
It is possible that your perception is influenced by a contextual feature in the samplke phrase: /tʃ+ð/ is likely to modify /tʃ/ to make is less retracted, making it sound more like ɕ.
Interjections like "ouch" are often an exception to the phonological system of a language, the words tsk and tut-tut-tut even contain clicks otherwise absent from the English phonology.
Examples from German include pfui (expression of disgust, a dictionary gives me the English translations "fie!" and "faugh!") and hui (expression of admiration, like "wow" or "great") with an otherwise unused diphthong, or mh-mh "no" with a clear tone contour.