“…the cumulative weight of poisoned nights and dirty-pale days”
Here’s a poem from just outside the nineteenth century (1906):
Ямбы
О, как я чувствую накопленное бремя
Отравленных ночей и грязно-бледных дней!
Вы, карты, есть ли что в одно и то же время
Приманчивее вас, пошлее и страшней!
Вы страшны нежностью похмелья, и науке,
Любви, поэзии — всему вас предпочтут.
Какие подлые не пожимал я руки,
Не соглашался с чем?.. Скорей! Колоды ждут…
Зеленое сукно — цвет малахитов тины,
Весь в пепле туз червей на сломанном мелке…
Подумай: жертву накануне гильотины
Дурманят картами и в каменном мешке.
I’m posting it because of line 11, which is strange for the same reason lines 3, 5, and 8 in Fedor Sologub’s “Men of the Eighties” (Восьмидесятники, 1892) are.* One more for my collection. Is this kind of rule-breaking everywhere, if I keep looking? Or just in early modernist poetry? Or in poems that call attention to formal elements (“Iambs” for a title) or have rebellion against classical education (and by extension neo-classical metrical rules) as a theme?
This poem is by Innokentii Annenskii (1855-1909), who I knew was much older than Viacheslav Ivanov, Kuzmin, or Blok, but I had forgotten was older than Chekhov. Here’s a quick prose translation:
Iambs
O, how I feel the cumulative weight of poisoned nights and dirty-pale days! You, cards — is anything at once more alluring, more vulgar, and more terrible than you!
You are terrible in your tender hangover, and people prefer you to science, love, poetry — everything. What villainous hands have I not shaken, what have I not agreed to…? Quick! The decks are waiting…
The green cloth is the color of malachite pond-scum, the ash-covered ace of hearts is on top of a broken piece of chalk… Think: the night before the guillotine, the condemned man finds cards intoxicating in the very dungeon.
* I think накануне ‘on the eve’ was sometimes written as two words (на канунѣ) in pre-revolutionary orthography (though more often not), but it looks like that was almost gone by 1906. (Also, it seems that Google Ngrams will miss some old words if you leave out a word-final hard sign, but you can search for “е” and get results that cover both “е” and “ѣ.”) Still, I suppose Annenskii could have treated the break between на and кануне as sufficient for the caesura, and then my list would be back down to just Sologub.
XIX век 
I’m not sure about Sologub but Annensky probably shifted the caesura to the left as a conscious technique. Like syncopation in music, it disrupts the regular rhythm and makes the ending somewhat more expressive. “Think of this!” and a deep breath, a gasp perhaps. So yes, it’s an early modernist formal trick.
“Iambs” rather than “Alexandrines” is a cop-out, just in case: you were warned it’s mere an iambic hexameter and Racine’s rules may not apply.
Is there a mere iambic hexameter in Russian verse, distinct from the alexandrine, though? I was taking “Iambs” as signaling that the poem contained invective (as well as drawing attention to the lapse in the observation of Racine-era rules, rather than declaring independence from them).
I like your interpretation of what Annenskii was doing with the caesura shift itself – I’m convinced.
Honestly, I could not come up with an example of a бесцензурный шестистопный ямб but web sources point out a poem by the Acmeist Vladimir Narbut and a poem with mixed hexameters and septameters by Alexander Blok. Disappointing…
…But I have just been rewarded for those nearly fruitless efforts. I’ve stumbled upon the PhD dissertation of none other than the young but relatively well-known Russian poet and critic Kirill Korchagin. “Цезура в русском стихе XVIII — первой четверти XX века” is the title. You can find it here. You will have to sign up or sign in with Facebook or Google+ but it shouldn’t be a problem.
“Heptameter”, not “septameter”, of course.
Thank you! I just downloaded Korchagin’s dissertation and look forward to reading it. The Blok poem is a good example. Some versions of the poem repeat тобой three times in line 10 and/or тебя three times in line 13, so that the third instance spoils an otherwise normal alexandrine, just as the third живу does in line 7. That makes it seem as if Blok is playing on what our ears expect, and also that (unless Blok published it both ways, which is possible – I haven’t checked) the writers of the book you linked to could not help normalizing the meter even when they were citing it as an exception.