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Napić się kieliszek koniaku [25]
"Napiję się kieliszka" sounds as if kieliszek were the liquid you want to drink. Really terrible!
ok. but napiję się kieliszek wódki sounds also weird.
You will see many mistakes made by Poles which are a result of wide spread for years errors which found their place in colloquial language and are well rooted there.
That should be applied then to "kieliszka case" because I've checked in google and many people write kieliszka as well as kieliszek. Strzyga is right - her justification sounds really logical. Still I hear more times szklanki, kieliszka if used in such sentence than szklankę mleka:
- napiłam się szklanki ciepłego mleka z miodem
rather than:
- napiłam się szklankę ciepłego mleka z miodem
So as you see even Poles make gramatical mistakes ;)