ANSWER - I think this is certainly possible. I've had some results myself on several occasions that have come from planned rather than actualised spell work and it is a bit of in joke between witches that on occasion we have reaped the rewards already from the spell scheduled to perform next week!
If you think about it, the act of working magic isn't that much different from focusing your thoughts. You are sending your desire into the world in the form of energy and spell work simply provides a structure for this to happen. But that isn't to say that such a structure is always necessary. Some spells are extremely simple - think candle magic where you are simply gathering and focusing your will - unconscious casting is just taking this a step further. I'd go so far as to say that there are many many people out there casting magic without even knowing it simply by holding a vision of the outcome they want in their mind and concentrating on it. A lot of what people actually put down to luck I see as being unconscious casting. (I'd actually go even further and invite you to consider the similarities with prayer and the many religions who consistently get similar results from praying. Not really much different to casting is it...)
So an unequivocal yes to this one. And I'd add that it is worth being aware of what you do focus your thoughts on. In a couple of cases I haven't had the heart to perform negative magic against certain individuals but I've been (enjoying) thinking about it enough to get the results anyway...