Watched my 3pm cold brew turn cloudy in a dirty French press I forgot to rinse at the office
Left the press sitting since Friday, poured my last 12 oz of concentrate in anyway, and it came out sour and gritty, so now I keep a backup jar of pre-made cold brew in the office fridge and rinse that thing the second I'm done.
That "sour and gritty" part is the real tell, because cloudy is just looks but sour means something actually grew in there. Old grounds sitting wet for a weekend turn into a science project, and no amount of pouring fresh concentrate over it will save the taste. A backup jar in the fridge is smart, but the rinse takes like ten seconds, so why not just do it right away instead of keeping a whole second setup going?