Like other teachers of elementary phonetics to NS students, when teaching I would regularly set exercises in “doing transcription”. I would give the students a passage of English in ordinary spelling. Their task would be to convert it into phonetic transcription.
This is a valuable exercise for NSs as much as for NNSs. It familiarizes them with the phonetic symbols. It makes them conscious of the difference between spelling and pronunciation. It alerts them to the characteristics of connected speech as opposed to individual words in isolation.
Good students sail through this task. The weaker ones often find it remarkably difficult. I would frequently have to point out that write does not actually begin with a w-sound, nor looked end with a d-sound (still less a syllabic d̩). The first vowel in particular is not normally pronounced ɑː, and the second vowel in information is not ɔː. Orthography has a distressingly dazzling effect on the phonetically unsophisticated.
But setting and marking (AmE ‘grading’) transcription can also be valuable for the teacher. Early in my career I noticed some students transcribing exist as ɪkˈzɪst (and similarly with example, exhausted, exams etc). Since the usual pronunciation is ɪɡˈzɪst I would mark this wrong. (Initial e-or ə- rather than ɪ- is OK, of course, though you have to check that that’s what they genuinely say, not just spelling-driven.)
However some students protested: they really do pronounce the word as ɪkˈzɪst. I checked it out, and they appeared to be correct. I came to realize that some speakers in the southeast of England, at least, have an unexpected dissimilation of voicing in these words. Their kz in exist seems to be different both from the gz of eggs or big zits and from the ks of exceed.
Being now a fortis plosive, it is also a candidate for glottal reinforcement: ɪkʔˈzɪst.
Convinced now of their reality, I decided to include these variants in LPD. But no other dictionary seems to recognize their ɪkˈzɪstəns.