I feel it is safe to say that outside of any context, когда-то defaults to meaning "at some point in the past", at least in the dialect I am speaking. I might even go so far as to say that that's the only meaning I use it in myself.
However, every once in a blue moon, I run into it being used to mean "at some point in the future". And it usually immediately sticks out. Even to the point of sounding ungrammatical.
I'm struggling to think of sample quotes right now, except for this rather famous one:
Детство кончится когда-то,
Ведь оно не навсегда.
Станут взрослыми ребята,
Разлетятся кто куда.
Now, that's none other than Entin right there, so I suppose he knew full well what he was doing. When I sing it I might not even notice or care. But when reading it, and stopping to think about it, I quickly realize that I really want that когда-то to be a когда-нибудь. Heck, even a когда-либо would be a more natural choice for me. At best, the когда-то feels like a one-off poetic-license type of thing; at worst, it makes me sound like a German spy blowing his cover.
Consequently, my question is: is this a one-off thing? Knowing the rules so you can break them? Or did I just happen to acquire a dialect where this is unusual, but other than that it is something well-attested going back hundreds of years? Perhaps even quite typical for other dialects (or registers)?
I would appreciate any pointers to research on the subject, or, failing that, a bunch of citations. While I mean no disrespect to Mr Entin, I'd be lying if I said that seeing this from someone of the caliber of a Lermontov or a Chekhov wouldn't be more convincing.