MeatSpace examples typically include the handling of lifestyle issues in the presence of parents or coworkers. For example, most parents would rather not believe that their teenage child smokes cigarettes (e.g.). This belief, in most cases, is more important than the truth. Therefore, most parents will choose to disregard most evidence that might suggest smoking. The teenager will oblige by refraining from flaunting the behavior (e.g. smoking), and will have a plausible story to explain away any problematic evidence (e.g. I was buying cigarettes for my friend's mom). The parent will refrain from any genuine attempts at investigation. Both child and parent benefit from a more civil environment. If both are perceptive, each will acknowledge to themselves exactly what is going on.
Examples exist in law enforcement. I once heard the story of a pilot who had the misfortune of crashing a small aircraft during landing, mainly due to carburetor icing. No one was hurt, and while the aircraft was destroyed, it was not particularly valuable and was not covered by insurance. The NTSB investigated, as it does with nearly all aviation accidents. Two empty portable fuel containers were present next to the aircraft amid baggage removed from the plane. Noting aloud that FAA regulations prohibit carrying even empty fuel containers in an aircraft, the investigator asked the pilot whether he had brought the fuel containers to the site after the crash. Was the pilot to suppose a truthful answer was expected? According to the story, the investigator went on to suggest that the containers be moved to another location before the wreckage was photographed, "to prevent misunderstandings."
Usually, there is no ethical dilemma in maintaining plausible deniability as long as the deceived party genuinely prefers not to know the truth. Difficulties arise only when facts come to light that cannot be, plausibly, overlooked.