The ‘cocktail party effect’ is the name given to our unusually adept ability of separating out conversations from one another. However it appears that we are unusually bad at retaining information from the discarded conversation(s):
Cherry  found his participants picked up surprisingly little information [from the ‘rejected’ conversations], often failing to notice blatant changes to the unattended message. When asked afterwards, participants:
- could not identify a single phrase from the speech presented to the rejected ear.
- weren’t sure the language in the rejected ear was even English.
- failed to notice when it changed to German.
- mostly didn’t notice when the speech to the rejected ear was being played backwards (though some did report that it sounded a bit strange).
via Mind Hacks