When I start making dumb mistakes OR when I start getting grumpy and the creative process becomes a battle of my will.
As for the dumb mistakes, it usually goes something like this: "Hm, I should really not do this because [statement of a bad thing that could happen] BUT I WILL BE CAREFUL.....and 9 times out of 10, I do "it" and the bad thing happens. Once I just needed to trim a little edge piece and cut right into a bunch of finished squares that happened to lie underneath. Most of the time I end up sewing the wrong sides/pieces together.
A tired mind also doesn't think creatively. Why did you want to use that particular fabric in the first place and what makes it inappropriate? Can you add a neutral border to make it work? I hate to throw in the towel before I tried all options.