I recently had a bad experience with reusing yeast from a primary fermenter. I had never done it before, but I figured it would be easy enough to just scoop up some yeast cells and store them in bottles in my fridge until my next brew day. It didn't go smoothly, some of the sludge got on my hands and I thought it may have contaminated the yeast. But rather than toss out the bottles, I thought I'd keep them for a bit as kind of an experiment to see what would happen to them. Well, a couple weeks later I was in the mood to brew, but didn't have any yeast packets. Rather than waste an hour and gas driving to the nearest brew store and back, I went against my better judgement and opted to use the yeast I had stored. I made a very basic extract beer since I knew there was a possibility I'd lose the batch. I warmed up the bottle of yeast in warm water, and then just added the yeast sludge to the wort in the fermenter (no starter..). Later in the day the airlock had begun to bubble away and I was starting to feel good about it. Proper sanitation? Starters? Eh, its all hype, I can't screw up a batch even if I try, right? Wrong. A few weeks later I racked to my bottling bucket and the beer looked like it always does. Nothing out of the ordinary. So I put my head close to the bucket and took a nice strong whiff. Wow. Burned my nose big time, it was like snorting vinegar (not that I've ever down that). Of course I had to call in my SWMBO and say "hey, will you smell my beer and tell me if you think it smells funny?... yeah...lean in close...." Oh man, you should have seen her reaction and her eyes watering up, I laughed my ass off. Needless to say, the whole batch went down the drain. But it wasn't a complete waste, I definately learned some valuable lessons. Everyone always talks about sanitation, but I think I needed to experience this first hand to truly understand why. And cutting corners isn't worth it, since then I've always kept extra yeast on hand. Hopefully my first and last infected batch.