I've dumped two, and they were both the same recipe (mild), but not because the recipe was bad. The first time it was 10 gallons of mild. I brewed it with a buddy who was visiting, and I hadn't been brewing in while and, honestly, kind of forgot about it. I remembered it 2 weeks later, but just didn't have time to tend to it, not to mention that I needed to clean up my kegs and keg storage to do it all properly... and then forgot again.
About a month later, I decided that a 6 week old mild sitting on a yeast cake was probably not going to be very good and wrote it off. I finally got around to dumping it about 6 months after that.
I'm back to brewing regularly again, though, and I tried that mild recipe again... and didn't hit my gravity. I didn't even realize it until I was running it off into my primary. So I let it go and, sure enough, I ended up with something that wasn't so hot, but here's the good part: I've recently started distilling and had some feints (the part of the distillation run you don't want to drink, but still has a considerable amount of ethanol it it), so I dumped the feints into the, uh, grain water, and distilled it. The hearts blended well and it's currently got another couple of weeks of oaking to go. I'm really looking forward to this!