By an "unobtainable shiny patch", you mean....?
As in, you're in a place where there are multiple places where grass is, and there are ledges, trees, etc. blocking the other parts.
The shiny patch shows up there, so you can't access it without cutting off your current chain.
well to be honest I saw this all in a website I always go to to see info about pokemon new episodes and it never failed me so..../:
it does seem logic (the plot) but now they erased that info so I am not too sure about it now...