Yeah this happens and nobody talks about it enough. I've had sessions where I'm convinced I caught something decent on the night, full sentence almost, play it back next morning and it sounds completely different - sometimes weaker, sometimes it's like a different voice entirely.
My mate thinks its just our ears being tired late at night making us hear patterns that aren't there. Could be partly true I suppose. But I've had the opposite happen too where I dismissed something on the night and the next day its clearly a word, no question about it.
Had one recording from a session at a place in Cannock Chase last year, three of us heard the same thing on playback that night. Next day two of us heard something completely different and the third lad couldn't hear anything at all anymore. Same file, no editing.
Don't think its purely psychological either. Anyone actually done any proper controlled testing on this or is it all just anecdotal across the board?