It's one of the most surprising things in my writing experience - reading through a manuscript and discovering a set of images you never intended to be there.
Actually, though, I'm of the opinion that it's been the norm for writers for quite some time. I extol the classics - Fielding, Dickens, Mary Elizabeth Braddon - but I don't honestly believe they had much more of a clue than we did. I can't deny they wrote fabulous novels but some of those things we pore over? Not completely deliberate.
I've just completed the final read-through of my manuscript. I was amazed as I read aloud how many references to graveyards and death I'd littered around the text without even noticing. Yes, my main location was a converted church but, beyond that, I had corpse imagery in a bar, spectral imagery in the street - it all seemed to mix together delightfully.
The key is to spot it early. You might've peppered a couple of recurring images throughout your WIP but if you recognise and grasp it you can easily enhance it, make it into a prolonged image that has a bearing on your main plot.
My preoccupation with death was more than just my brain being creepy. My protagonist was a bit of a recluse, living in this converted church and painting constantly. Although it wasn't a deliberate decision to coat the text with graveyard imagery it certainly fits with her personality and the dilemma her love interest has in trying to prise her out of there.
Once I spotted it I had to run with it. I can't be sure - I accept I'm too close to this novel at the moment - but I think it enhances the overall effect. I do love my subconscious sometimes.