The 161m (0.1mile) rule in Geocaching is a pretty infamous one. However, the rules also state this:
Please don't hide a cache every 600 feet just because you can. The two main goals of the saturation guideline are to encourage you to seek out new places to hide caches rather than putting them in areas where caches already exist, and to limit the number of caches hidden in a particular area, especially by the same hider. Groundspeak may further restrict cache listings in areas where cache saturation becomes a concern.
So when exactly does an area become saturated and need further restriction? I've come across lots of areas with caches every 161m or so, some ridiculously riddled with caches. Is there any example of this rule being put into practice?