I used to listen to a lake background noise sound, I would just have it on for hours while programming or relaxing. But at some point I heard that there was a little plopping sound, just some water that splashed in a certain way to produce that sound.

And soon I heard it again, and again, and again. I knew the audio looped, but having detected a distinguishing sound like that plop broke the illusion for me completely, I was no longer listening to the calm waters of a lake, but a looping wav file.