For what it is worth, I had a two year old slurry in the back of the fridge in a plastic tub with a loose lid and a paper towel as a "filter-like" medium with an inch or so of beer on top of the yeast cake. Other than some dead yeast at the topmost part of the slurry, it was without any infection and as I pored it out to make room for a fresh slurry to take its place, I was amazed that it smelled so fresh. I wouldn't re-pitch it, of course, but I would not hesitate making a starter using it in that condition. I would also say that when handling yeast - try to have everything to be used in the process all laid out and work quickly with open containers containing yeast to avoid airborne contaminants getting in the medium.
There is nothing worse than discovering a defect on a beer from which a slurry was harvested and already re-pitched, because then you have two batches affected...I've learned the hard way. Some defects don't show up right away and get intolerable with time. It doesn't prevent me from re-pitching, it just makes me cautious about cleaning, sanitizing and transferring.