Is /sʌbˈsiːkwənt/ a correct pronunciation of the word "subsequent"

Usual pronunciation of subsequent is /ˈsʌbsɪkwənt/, or maybe /ˈsəbsəkwənt/.

However, today I heard an educated person pronouncing it /sʌbˈsiːkwənt/.

Is this a correct pronunciation?

As far as I know, pronouncing "subsequent" with stress on the second-to-last syllable has never been generally considered correct. Merriam-Webster, commonly considered a relatively "permissive" dictionary in terms of pronunciation, does not list it, and the old classic of prescriptive pronunciation, Walker's Critical Pronouncing Dictionary (which can be useful as a resource for archaic pronunciations, since the linked edition is from 1791) gives stress on the third-to-last syllable without any mention of other variants. The pronunciation with the accent on the third-to-last syllable is also the only one given by Macmillan, Oxford Dictionaries, the Cambridge Dictionary, the American Heritage Dictionary, and the Collins dictionary.

English stress placement is complex, and I don't know of any argument for considering /sʌbˈsiːkwənt/ "incorrect" that is more compelling than the simple fact that most people don't use it. However, because I find it interesting, I'll attempt to describe the "rules" for stress placement in words of this type. I'm just an interested amateur, so some of this may not be fully correct.

Antepenult stress is, in general, a common pattern for English tri- and polysyllables. Some tri- or polysyllabic words ending in -ent or -ant are always or sometimes stressed on the second-to-last (penult) syllable, but typically this occurs only for words that meet one or more of certain phonological, etymological and morphological criteria. "Subsequent" only meets the morphological criterion, which is somewhat shaky.

• Phonological motivation for penult stress: Penult stress occurs most regularly in words of this type when there is a certain kind of consonant cluster after the stressed vowel (as in consistent). But "qu" does not typically count as a stress-attracting consonant cluster (compare consequent and eloquent). Even so, the fact that /kw/ is a consonant cluster might slightly facilitate a variant pronunciation with stress on the second syllable (for comparison, even though "gr" is not typically a stress-attracting consonant cluster, certain speakers are known to pronounce the word "integral" as /ɪnˈtɛgrəl/ or /ɪnˈtiːgrəl/, with penult stress.)

• Etymological motivation for penult stress: Less regularly, penult stress can occur when the vowel was long or a diphthong in Latin (as in antecedent, precedent, decedent; however, precedent has an alternative pronunciation with stress on the third-to-last syllable). This reason for penult stress seems somewhat artificial (see the Sargeaunt quote below) and in fact, penult stress is not used in all -ent/-ant words that had a long vowel in Latin. In any case, the second-to-last "e" of subsequent was not long in Latin (it comes from Latin sŭbsĕquēns).

• Morphological motivation for penult stress: Sometimes, penult stress seems to occur just because the third-to-last syllable is (part of) a prefix, as in supernatant /ˌsuːpərˈneɪtənt/, from Latin sŭpĕrnătāns = sŭpĕr-nătāns, and covalent, from co- + valent from Latin vălēns.

  This might be why the person you heard said /sʌbˈsiːkwənt/: it may be meant to represent the structure of the word as "sub-sequent". Sometimes people consciously choose to use unconventional stress to draw attention to the structure of a word: I had a science teacher in high school who liked to pronounce "hydrolysis" as /haɪdroʊˈlaɪsɪs/ in class to bring our attention to the fact that it meant "splitting by water".

  However, it is possible and common for an antepenult syllable that was a prefix in Latin to receive the primary stress/accent in an -ant/-ent adjective—as shown by words like deferent, different, abstinent, competent, innocent, indolent, insolent, applicant—and this is arguably the more regular stress pattern (and the one that the most common pronunciation of subsequent follows).

Some useful comments by rjpond point out that some dictionaries, such as the Oxford English Dictionary and Webster's Third, do record a pronunciation of the related noun "subsequence" with the accent on the first syllable, but an unreduced/secondarily stressed vowel /iː/ in the second syllable.

Here is what John Sargeaunt had to say about the pronunciation of words of this type in The Pronunciation of English Words Derived from the Latin (1920):

Stems ending in -ant and -ent. These are participles or words formed as such. Our words have shed a syllable, thus regentem has become 'regent'. Disyllables follow the 'apex' rule and lengthen the first vowel, as 'agent', 'decent', 'potent'. Exceptions are 'clement' and 'present', perhaps under French influence. Words of more than two syllables with a single consonant before the termination throw the stress back and shorten a long penultima, as 'ignorant', 'president', 'confident', 'adjutant'. Where there are two heavy consonants, the stress remains on the penultima, as 'consultant', 'triumphant', even when one of the consonants is not pronounced, as 'reminiscent'. In some cases the Latinists seem to have deliberately altered the natural pronunciation. Thus Gower has 'ápparaúnt', but the word became 'appárent' before Shakespeare's time, and later introductions such as 'adherent' followed it. What right 'adjacent' has to its long vowel and penultimate stress I do not know, but it cannot be altered now.

I could imagine such a pronunciation in mathematical logic. A sequent is a certain kind of conditional assertion. Accented on the se. Perhaps someone will one day define something contained in a sequent, calling it a sub-sequent .... and if it is frequently used it would become to be spelled subsequent. But accented on the se.

Mathematicians already have the word subsequence accented on the se